---
title: "From Egg Cell to Birth"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 58
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/58-from-egg-cell-to-birth
---

# Chapter 58 — From Egg Cell to Birth

[Chapter 32](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#ch-g5-human-reproduction) told the nine months as a story; you now own every instrument the story needs played properly: [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) and their divisions, the two [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) and their meeting place, [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones), the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)’s deliveries. This chapter replays human development with the machinery showing — from one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) to three thousand billion, in forty weeks.

## 58.1 The first week: from zygote to lodger

**Proposition 58.1 (The journey down the oviduct).**

[Fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) in the [oviduct](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-female) ([Definition 57.3](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-female)) starts two clocks at once:

1. *division* : the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) divides within a day, then again and again ( [Example 29.8](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ex-g5-first-look-at-cells-one) ) — two, four, eight [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , a mulberry-ball of dozens by day four, all while drifting down the [oviduct](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-female) toward the uterus;
2. *implantation* : around day seven the little ball reaches the uterus and embeds itself in the prepared lining ( [Proposition 57.5](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#prop-g8-reproductive-systems-cycle) ’s event 4) — the lodger arrives, the cycle stops, and [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) signal the whole system: [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) .

**Proof.** *Admitted at this level.* ∎

**Example 58.2 (What the first divisions live on).**

Through the first week the traveler eats no meal and receives no delivery: it runs on the [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)’s own provisions — exactly what [Example 57.4](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#ex-g8-reproductive-systems-egg)’s stocking was for. The sperm brought the father’s [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) and little else; the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) brought the other [nucleus](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#def-g6-cell-unit-of-life-parts) *and* the pantry. Implantation is, among other things, the pantry running low and the lodger plugging into room service.

## 58.2 Embryo, placenta, fetus

**Definition 58.3 (The placenta).**

From the implanted ball grow two constructions at once: the *[embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)* itself, and its life-support interface — the *placenta*, a disc grown into the uterus wall where the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) and the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) come intimately close *without mixing*. Across its vast thin surfaces — the specification of [Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec), built once more — [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) cross to the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy), wastes and [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) cross back ([Proposition 32.4](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#prop-g5-human-reproduction-cord)’s cord runs between placenta and [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)). The placenta is [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), intestine and [kidney](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) to the passenger — all three borders in one organ.

**Proposition 58.4 (The timetable, revisited with instruments).**

[Example 32.5](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#ex-g5-human-reproduction-timetable)’s milestones, now explained:

- weeks 1–8, the *[embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)* : the body plan is laid out — [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) divide, [migrate](https://one-course.com/books/biology/1/en/chapter/20-animals-through-the-seasons#def-g3-animals-through-seasons-migration) and *specialize* ( [Remark 45.6](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#rem-g6-cell-unit-of-life-twoways) ) into [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) , [nerve](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) , [bone](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) and the rest; the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) beats from week four, because the pantry-and-placenta economy needs its pump early;
- weeks 9–40, the *[fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)* : the plan built, growth and ripening — the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) wiring itself, [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) rehearsing empty ( [Exercise 32.11](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#exo-g5-human-reproduction-11) explained their first-cry debut), the kicks of month four announcing working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ;
- throughout: [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) hold the uterus quiet and the cycle suspended — the chemistry of [Definition 56.4](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) , running [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) ’s administration.

**Proof.** *Admitted at this level.* ∎

**Example 58.5 (Why the early weeks ask the most care).**

[Proposition 32.8](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#prop-g5-human-reproduction-care) warned that tobacco and alcohol cross to the baby; the timetable says when it matters most: in the plan-laying [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) weeks, when each organ is a small crowd of specializing [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), one disturbed instruction can echo through the whole build — and those are precisely the weeks a [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) can pass unnoticed. Hence the doctors’ blunt arithmetic: care begins not at the first scan but at the first possibility.

## 58.3 Birth, and the changeover

**Proposition 58.6 (Birth as a hormone-ordered changeover).**

Around week forty, [hormone](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) signals — the baby’s and the mother’s in dialogue — end the uterus’s long quiet:

1. *labor* : the uterus, the body’s strongest [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) ( [Remark 57.6](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#rem-g8-reproductive-systems-living) met its gentler work), contracts in strengthening waves that open the way and push the baby down the [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) canal;
2. *[birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle)* : usually head first ( [Example 32.6](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#ex-g5-human-reproduction-birth) );
3. *the changeover* : the first cry fills the never-used [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) ; the cord’s traffic ends and it is cut; within minutes the newborn runs on its own [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) , gut-to-come and [kidneys](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) — every border the [placenta](#def-g8-from-egg-to-birth-placenta) had covered ( [Exercise 32.11](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#exo-g5-human-reproduction-11) , now with the full map);
4. *afterbirth* : the [placenta](#def-g8-from-egg-to-birth-placenta) , job done, detaches and follows — the only organ the body builds for one use and discards.

**Proof.** *Admitted at this level.* ∎

**Example 58.7 (The newborn’s first audit).**

Midwives score a newborn minutes after [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) — [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), [heartbeat](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), color, movement, cry. Read the checklist with this book’s [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): it is [Proposition 53.7](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#prop-g7-removing-wastes-complete)’s supply-and-removal loop, audited at its first solo run. The exam every human passes first is a physiology test.

**Method 58.8 (Explaining any stage of pregnancy).**

Asked about any moment of the nine months, answer with four instruments:

1. *[cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)* : what are the divisions and specializations doing?
2. *supply* : pantry, or [placenta](#def-g8-from-egg-to-birth-placenta) — what crosses, what must not?
3. *[hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones)* : what is being held, or ordered?
4. *care* : what does the stage ask of the mother’s habits and the doctors’ calendar?

## 58.4 Exercises

**Exercise 58.1 ★.**

Time the first week: division’s start, the mulberry ball, implantation.

**Solution of Exercise 58.1.**

Division within a day of [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps); a mulberry-ball of dozens of [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) by day four, drifting down the [oviduct](https://one-course.com/books/biology/1/en/chapter/57-the-reproductive-systems#def-g8-reproductive-systems-female); implantation in the uterus lining around day seven.

**Exercise 58.2 ★.**

What does the first week’s traveler live on, and what ends that arrangement?

**Solution of Exercise 58.2.**

On the [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)’s own provisions — the pantry the big [gamete](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) was stocked as. Implantation ends the arrangement: the lodger plugs into the lining’s supply, soon organized as the [placenta](#def-g8-from-egg-to-birth-placenta).

**Exercise 58.3 ★.**

What is the [placenta](#def-g8-from-egg-to-birth-placenta) — what crosses it each way, and what never crosses?

**Solution of Exercise 58.3.**

The life-support disc grown into the uterus wall, where the two [bloods](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) come close without mixing. To the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy): [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). Back: wastes and [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). Never crossing: the [bloods](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) themselves.

**Exercise 58.4 ★.**

Divide the forty weeks into [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) and [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy), with each phase’s business.

**Solution of Exercise 58.4.**

Weeks 1–8, the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy): the body plan laid out by dividing, migrating, specializing [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). Weeks 9–40, the [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy): the plan built — growth and ripening, [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) wiring, [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) rehearsing.

**Exercise 58.5 ★.**

Why does the [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) beat from week four, of all organs first?

**Solution of Exercise 58.5.**

Because the whole build runs on deliveries: as soon as the [placenta](#def-g8-from-egg-to-birth-placenta) economy starts, a pump must move the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy)’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) through cord and body — the supply line cannot wait for the rest of the plan.

**Exercise 58.6 ★.**

List [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle)’s four numbered events in order.

**Solution of Exercise 58.6.**

Labor’s strengthening contractions; [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), usually head first; the changeover — first cry, cord cut, the newborn’s own borders taking over; the afterbirth — the [placenta](#def-g8-from-egg-to-birth-placenta) detaching and following.

**Exercise 58.7 ★★.**

Which three organs’ jobs does the [placenta](#def-g8-from-egg-to-birth-placenta) do? Name the border chapter that covered each.

**Solution of Exercise 58.7.**

The [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ ([oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) out — [Chapter 51](https://one-course.com/books/biology/1/en/chapter/51-the-lungs-and-gas-exchange#ch-g7-lungs-and-gas-exchange)), the intestine’s ([nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) in — [Chapter 50](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#ch-g7-digestion-and-nutrients)), and the [kidneys](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys)’ (wastes out — [Chapter 53](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#ch-g7-removing-wastes)). Three borders, one disc.

**Exercise 58.8 ★★.**

Why do the [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) weeks ask the most care — two reasons from [Example 58.5](#ex-g8-from-egg-to-birth-earlycare)?

**Solution of Exercise 58.8.**

Because the plan-laying weeks are when one disturbed instruction echoes through the whole build — each organ is a small crowd of specializing [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell); and because those weeks can pass before the [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) is even suspected — so care must begin at the first possibility, not the first scan.

**Exercise 58.9 ★★.**

Read the newborn’s score sheet as [Proposition 53.7](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#prop-g7-removing-wastes-complete)’s loop: match [breathing](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration), [heartbeat](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) and color to their systems.

**Solution of Exercise 58.9.**

[Breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) and the cry: the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ border, minutes old. [Heartbeat](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart): the pump driving both circuits. Color: the delivery service actually delivering — oxygen-rich [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) reaching the [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’s [capillaries](https://one-course.com/books/biology/1/en/chapter/52-the-heart-and-circulation#def-g7-heart-and-circulation-vessels). Movement completes the commander’s check. The score is the loop, audited live.

**Exercise 58.10 ★★.**

Identical twins revisited ([Remark 32.7](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#rem-g5-human-reproduction-twins)): at which point of [Proposition 58.1](#prop-g8-from-egg-to-birth-firstweek) does the [one-zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core) split happen, and why does the timing make them “alike as copies”?

**Solution of Exercise 58.10.**

In the first divisions — the [zygote](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#prop-g8-sexual-reproduction-core)’s ball splitting into two balls within the first days. Both twins then build from the same starting [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s inheritance ([Example 29.8](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ex-g5-first-look-at-cells-one)), so the two builds run the same plans: copies, because the fork came after the deal.

**Exercise 58.11 ★★.**

Run [Method 58.8](#met-g8-from-egg-to-birth-explain) on month four — the month of the first kicks.

**Solution of Exercise 58.11.**

[Cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): the [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) phase — organs built, now growing; [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and [nerve](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) wiring together. Supply: [placenta](#def-g8-from-egg-to-birth-placenta) economy at full run — [nutrients](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) and [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) in, wastes out, nothing harmful allowed in the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb). [Hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones): holding the uterus quiet, the cycle suspended. Care: the mother’s plate and rest feeding two borders’ worth of traffic; the kicks themselves are the reassuring sign of the working loop.

**Exercise 58.12 ★★★.**

“[Birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) is a change of borders.” Justify: list every exchange the [placenta](#def-g8-from-egg-to-birth-placenta) handled, and the organ that takes each over at the first cry — with the minute-scale timing that makes the changeover remarkable.

**Solution of Exercise 58.12.**

The [placenta](#def-g8-from-egg-to-birth-placenta) handled [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) loading, [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) removal, [nutrient](https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients#def-g7-digestion-and-nutrients-families) supply and waste clearance. At the first cry the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) take the two gases; the gut (from the first feeds) takes supply; the [kidneys](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) take clearance — and all within minutes of the cord’s cutting, a border handover that every organ must pass on its first try, which is exactly what the delivery room’s checklist watches.

## 58.5 Problem: The Midwife’s Timeline

**Problem 58.1.**

Weekend problem — a pregnancy followed visit by visit

A midwife’s file for one (invented) [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy): booking visit at week 8, scans at weeks 12 and 20, checks at 28 and 36, [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) at week 40. Read the file with the chapter’s instruments.

**Part I — The file’s early pages.**

1. The [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) was confirmed by a [hormone](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) test weeks before the first visit. Which signal does such a test read, and from which event of [Proposition 58.1](#prop-g8-from-egg-to-birth-firstweek) onward does it exist?
2. At week 8 the midwife records “ [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) complete in plan, about the size of a bean”. Translate into the timetable’s terms — what phase closes here?
3. The booking visit’s advice leads with no alcohol, no smoke — and folic acid, a builder’s vitamin, “ideally from before the start”. Justify all three from [Example 58.5](#ex-g8-from-egg-to-birth-earlycare) .
4. Week 12’s scan shows the [heartbeat](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart) that began at week 4. Why so early, in supply terms?

**Part II — The middle months.**

5. Week 20’s scan measures every organ and the [placenta](#def-g8-from-egg-to-birth-placenta) ’s position. Give the [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) phase’s business, and the [placenta](#def-g8-from-egg-to-birth-placenta) ’s three jobs being checked.
6. The mother reports the first kicks near week 18. What machinery is announcing itself ( [Proposition 33.2](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#prop-g5-brain-in-command-loop) names the chain)?
7. At week 28 the midwife checks the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) for sugar and pressure. Why is the mother’s circulation doubly loaded — whose borders is it running?
8. Weeks 30–36: the [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) mainly grows and the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) rehearse. Rehearse what — and for which single moment?

**Part III — The last page.**

9. Week 40: labor. Name the [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) , its two tasks, and who ordered the work ( [Proposition 58.6](#prop-g8-from-egg-to-birth-birth) ).
10. The file’s [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) entry: “strong cry at once; cord clamped and cut; [placenta](#def-g8-from-egg-to-birth-placenta) complete.” Audit the three clauses with the changeover.
11. The newborn scores high on the minutes-scale checklist. State what the checklist certifies, in [Example 58.7](#ex-g8-from-egg-to-birth-audit) ’s terms.
12. Close the file with one sentence for the parents: what the forty weeks built, from what single [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , by which machinery.

**Solution of Problem 58.1.**

**1.** The [pregnancy](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones) that begin with implantation — the settled lodger’s chemical signal; from about day seven on, they are there to read.

**2.** The [embryo](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) phase closes: weeks 1–8 laid the complete plan — every organ begun, bean-sized. [Fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) phase — building and ripening — begins.

**3.** Alcohol and smoke cross to the passenger, and the plan-laying weeks are the most vulnerable to disturbed instructions; folic acid is material the earliest construction draws on — and since those weeks can pass unnoticed, the vitamin’s “from before the start” is the same arithmetic as the abstentions.

**4.** The [placenta](#def-g8-from-egg-to-birth-placenta) economy needed its pump from the start: deliveries cannot circulate without a beating [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), so it is the first machine turned on — and the scan’s flicker at week 12 is it, eight weeks into its run.

**5.** The [fetus](https://one-course.com/books/biology/1/en/chapter/32-human-reproduction#def-g5-human-reproduction-pregnancy) phase: growth and ripening of the laid plan. The [placenta](#def-g8-from-egg-to-birth-placenta) is checked as [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) (gas exchange), intestine (supply) and [kidney](https://one-course.com/books/biology/1/en/chapter/53-removing-the-wastes#def-g7-removing-wastes-kidneys) (clearance) — the passenger’s three borders in one disc, plus its position clear of the exit.

**6.** The loop of report-decide-order: [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) contracting on [nerve](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) orders — the commander’s wiring and its motors, announcing themselves through the uterus wall.

**7.** Because it serves two customers’ borders: her own body’s, and — through the [placenta](#def-g8-from-egg-to-birth-placenta)’s traffic — the passenger’s. Sugar and pressure read how the doubled load is being carried.

**8.** [Breathing](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-movements) movements with no air — the bellows practicing empty, [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and reflexes readying for the single moment of the first cry, when the organ must work on its first try.

**9.** The uterus — the body’s strongest [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle); its tasks: opening the way, and pushing the baby down the canal; the orders came as [hormones](https://one-course.com/books/biology/1/en/chapter/56-puberty#def-g8-puberty-hormones), the baby’s and mother’s chemistry in dialogue.

**10.** The cry: [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) filled and working — border one transferred. The cord clamped and cut: the [placenta](#def-g8-from-egg-to-birth-placenta)’s traffic ended. The [placenta](#def-g8-from-egg-to-birth-placenta) complete: the one-use organ fully detached — the changeover closed with nothing left behind.

**11.** That the supply-and-removal loop — [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four), pump, deliveries, commander — runs solo: the first audit of the machine this book spent two grades mapping.

**12.** “Forty weeks built three thousand billion specialized [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) from one fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — by division, specialization and hormone-run administration, supplied across one remarkable disc.”