--- title: "Human Reproduction" book: "Primary & Middle School Biology" subject: biology language: en chapter: 32 exercises: 11 source: https://one-course.com/books/biology/1/en/chapter/32-human-reproduction --- # Chapter 32 — Human Reproduction Every person you know — friends, teachers, the oldest person in your family — arrived the same way: as a baby, born after months of growing inside their mother. Humans are [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) ([Proposition 21.4](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#prop-g4-classifying-animals-classes)), and human reproduction runs on the mammal plan of [Chapter 23](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ch-g4-animal-reproduction) — with the numbers, dates and details of our own [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species), which this chapter sets out plainly. ## 32.1 The human version of the plan **Proposition 32.1 (Human reproduction is mammal reproduction).** Like all [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), humans reproduce sexually ([Definition 23.1](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)): - the mother’s body produces *[egg cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)* ; the father’s produces *[sperm cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)* ; - [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) — one [sperm cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) joining one [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — happens *inside the mother’s body* ( [Proposition 23.6](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-where) ); - the fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — one single [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ( [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) ) — settles in the mother’s *womb* and grows there; - humans are [viviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) ( [Definition 23.4](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) ): the baby is [born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) , and is fed at first on [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) . **Proof.** *Admitted at this level.* ∎ **Remark 32.2 (Reproduction needs grown-up bodies).** A child’s body cannot make babies: the reproductive organs only start working at *puberty*, the change of [Definition 15.1](https://one-course.com/books/biology/1/en/chapter/15-the-stages-of-life#def-g3-stages-of-life-stages)’s [adolescent](https://one-course.com/books/biology/1/en/chapter/15-the-stages-of-life#def-g3-stages-of-life-stages) stage, when the body rebuilds itself from a child’s into an adult’s. What puberty changes, and how, is the subject of a full chapter in the middle-school years of this book. ## 32.2 Nine months in the womb **Definition 32.3 (Pregnancy).** *Pregnancy* is the time the future baby spends growing in its mother’s [womb](#prop-g5-human-reproduction-plan): about *nine months* in humans. The growing baby is first called an *embryo*, then — once its body plan is laid out, after about two months — a *fetus*. **Proposition 32.4 (Fed through the cord).** Inside the [womb](#prop-g5-human-reproduction-plan), the baby floats in a pocket of liquid and cannot eat or breathe. Everything comes ready-made from the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb): [nutrients](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) and oxygen ([Proposition 27.1](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-carries)) cross into the baby’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) through the *umbilical cord* at its belly — and its wastes cross back the same way, for the mother’s body to deal with. Your navel is where your own cord was attached. **Proof.** *Admitted at this level.* ∎ **Example 32.5 (The nine-month timetable).** Month 1: a cluster of dividing [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), smaller than a grain of rice, but already growing fast. Month 2: an [embryo](#def-g5-human-reproduction-pregnancy) with the sketch of a head, a beating [heart](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-heart), buds for [arms](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) and [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) — still smaller than a thumb. Months 3–4: a [fetus](#def-g5-human-reproduction-pregnancy), complete in plan, that begins to move; the mother soon feels the kicks. Months 5–8: growing and ripening — hearing arrives, and a first coat of hair. Month 9: the baby, usually head-down and around $3\,\mathrm{kg}$, is ready. **Example 32.6 (Birth).** At [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), powerful [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) of the [womb](#prop-g5-human-reproduction-plan) push the baby out into the world, usually head first. The first cry fills the [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) with air — they had never breathed before ([Proposition 32.4](#prop-g5-human-reproduction-cord)) — the cord, no longer needed, is cut painlessly, and the mammal plan continues outside: [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), warmth and care ([Example 23.9](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ex-g4-animal-reproduction-whalecalf) would approve). ![Days old, and the grip already works: the long human childhood begins.](https://one-course.com/images/onecourse/chapters/biology-1/g5-human-reproduction/img-469d09bbc903.jpg) *Days old, and the grip already works: the long human childhood begins.* **Remark 32.7 (Twins).** Sometimes two babies grow in the [womb](#prop-g5-human-reproduction-plan) together. If one fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) split into two [embryos](#def-g5-human-reproduction-pregnancy), the twins are *identical* — built 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), alike as two copies. If two [egg cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) were fertilized by two [sperm cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), the twins are no more alike than ordinary siblings — they only shared the [womb](#prop-g5-human-reproduction-plan). Two kinds of twins, told apart by their beginning. ## 32.3 Care before and after **Proposition 32.8 (The baby shares its mother’s supplies).** Everything reaching the baby travels through the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) — which is why an expecting mother eats well ([Chapter 18](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#ch-g3-balanced-diet)), rests, is followed by doctors or midwives, and avoids what could harm the baby: tobacco smoke and alcohol pass through the cord too, and a [fetus](#def-g5-human-reproduction-pregnancy) is far more fragile than an adult. Caring for the mother *is* caring for the baby. **Proof.** *Admitted at this level.* ∎ **Example 32.9 (A very cared-for species).** Even among [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), humans push the few-and-cared-for strategy of [Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies) to the extreme: usually one baby at a time, helpless far longer than a foal or a kitten, and cared for — fed, taught, loved — for many *years*. Human childhood is one of the longest in the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) world; this book, a decade-long course, is itself a piece of that long care. ## 32.4 Exercises **Exercise 32.1 ★.** Which [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) comes from the mother, which from the father, and what is their joining called? **Solution of Exercise 32.1.** The [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) from the mother, the [sperm cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) from the father; their joining is [fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps). **Exercise 32.2 ★.** Where does the human baby grow, and for about how long? **Solution of Exercise 32.2.** In the mother’s [womb](#prop-g5-human-reproduction-plan), for about nine months — the [pregnancy](#def-g5-human-reproduction-pregnancy). **Exercise 32.3 ★.** What are the growing baby’s two names, in order, and roughly when does the first become the second? **Solution of Exercise 32.3.** [Embryo](#def-g5-human-reproduction-pregnancy) first, then [fetus](#def-g5-human-reproduction-pregnancy) — from about two months, once the body plan is laid out. **Exercise 32.4 ★.** The baby in the [womb](#prop-g5-human-reproduction-plan) neither eats nor breathes. How does it get [nutrients](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) and oxygen, and where do its wastes go? **Solution of Exercise 32.4.** Through the [umbilical cord](#prop-g5-human-reproduction-cord): [nutrients](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) and oxygen cross from the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) into the baby’s, and the baby’s wastes cross back for the mother’s body to deal with. **Exercise 32.5 ★.** What is your navel the mark of? **Solution of Exercise 32.5.** The place where your own [umbilical cord](#prop-g5-human-reproduction-cord) was attached, before it was cut at [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle). **Exercise 32.6 ★.** What happens at the first cry, and why is it such a change for the baby’s body? **Solution of Exercise 32.6.** The first cry fills the [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) with air for the first time: until that moment they had never breathed — oxygen had always arrived through the cord. **Exercise 32.7 ★.** Which features make human reproduction typically mammalian? Name three, citing [Proposition 21.4](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#prop-g4-classifying-animals-classes) or [Chapter 23](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#ch-g4-animal-reproduction). **Solution of Exercise 32.7.** [Fertilization](https://one-course.com/books/biology/1/en/chapter/31-from-flower-to-fruit#def-g5-flower-to-fruit-steps) inside the mother’s body; a [viviparous](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-ovivivi) [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) (the baby grows in the [womb](#prop-g5-human-reproduction-plan) and is [born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)); [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) feeding afterward — the mammal class features exactly. **Exercise 32.8 ★.** Why do doctors ask expecting mothers to avoid tobacco smoke and alcohol? Use [Proposition 32.8](#prop-g5-human-reproduction-care). **Solution of Exercise 32.8.** Because whatever is 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) reaches the baby through the cord — smoke’s and alcohol’s harmful substances included — and a [fetus](#def-g5-human-reproduction-pregnancy) is far more fragile than an adult. **Exercise 32.9 ★★.** Explain the two kinds of twins by their beginnings. Which kind is alike “as two copies”, and why does that follow from [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)? **Solution of Exercise 32.9.** Identical twins: one fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) split into two [embryos](#def-g5-human-reproduction-pregnancy). Ordinary (fraternal) twins: two [egg cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) fertilized by two [sperm cells](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), sharing only the [womb](#prop-g5-human-reproduction-plan). The identical pair are alike as copies because a body is built from its first [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — and they started from the same one. **Exercise 32.10 ★★.** Place the human on the strategy line of [Proposition 23.7](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#prop-g4-animal-reproduction-strategies), comparing with the codfish and the elephant: number of young, length of care, and what that long care contains for humans. **Solution of Exercise 32.10.** At the elephant’s end, and beyond it: usually one baby, years of helplessness, and care that includes not only feeding and protection but teaching — language, skills, school. The codfish’s millions-with-no-care is the opposite pole. **Exercise 32.11 ★★★.** At [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle), the cut cord ends the baby’s supply line at the very moment the [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) take their first breath. Using [Proposition 32.4](#prop-g5-human-reproduction-cord) and [Proposition 26.5](https://one-course.com/books/biology/1/en/chapter/26-breathing#prop-g4-breathing-exchange), explain what job switches, in that minute, from the mother’s body to the baby’s own [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) — and why the first cry is well timed. **Solution of Exercise 32.11.** The job of loading oxygen and unloading carbon dioxide: until [birth](https://one-course.com/books/biology/1/en/chapter/7-born-growing-dying#def-g2-born-grow-die-cycle) the mother’s [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) did it through the cord; from the first breath, the baby’s own [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs)’ air pockets take it over. The cry is well timed because it comes at the exact minute the old supply line is cut — the [lungs](https://one-course.com/books/biology/1/en/chapter/26-breathing#def-g4-breathing-organs) must start precisely when the cord stops.