Pregnancy & Human Development
Fertilization, implantation, the placenta, embryonic and fetal development, labor and lactation
By the end of this unit you can…
- ✓Describe sperm capacitation, the acrosomal reaction, fertilization and blocks to polyspermy
- ✓Describe cleavage, blastocyst formation and implantation
- ✓Explain how the placenta forms and functions, and the role of hCG
- ✓Describe gastrulation, the three germ layers and their derivatives, and neurulation
- ✓Describe fetal circulation and the changes at birth
- ✓Describe maternal changes, the stages of labor and the hormonal control of birth and lactation
Key terms
Practice →1Fertilization
Gestation lasts about 40 weeks from the last menstrual period (~38 weeks from fertilization). The conceptus is called an embryo from fertilization through week 8 and a fetus from week 9 to birth.
Timing matters: the ovulated secondary oocyte is viable for only 12–24 hours; sperm survive ~3–5 days in the female tract. Sperm must undergo capacitation in the female tract — their membranes are altered so they become fully motile and able to release acrosomal enzymes.
- 1Sperm swim through the corona radiata (follicle cells) surrounding the oocyte.
- 2Acrosomal reaction: sperm bind to receptors in the zona pellucida and release acrosomal enzymes that digest a path through it — many sperm are needed to clear the way.
- 3One sperm's membrane fuses with the oocyte membrane; its nucleus enters.
- 4Blocks to polyspermy: the cortical reaction releases enzymes that harden the zona pellucida and remove sperm receptors, so no other sperm can enter.
- 5The secondary oocyte completes meiosis II (ovum + second polar body). The male and female pronuclei fuse → a diploid zygote (2n = 46).
2Cleavage & implantation
Cleavage is a series of rapid mitotic divisions without growth — cells (blastomeres) get smaller with each division. By day 3–4, a 16+ cell ball called the morula reaches the uterus. By day 4–5 it becomes a fluid-filled blastocyst.
- Trophoblast
- Outer layer of the blastocyst — forms part of the placenta (chorion); secretes hCG.
- Inner cell mass
- Cluster on one side — becomes the embryonic disc and so the embryo itself (plus amnion and yolk sac).
- Zona pellucida
- Shed (“hatching”) before the blastocyst implants.
Implantation begins about 6–7 days after ovulation: the trophoblast adheres to the endometrium and its cells form the invasive syncytiotrophoblast, which digests its way in; the endometrium then grows over the blastocyst. Implantation is complete by about day 12.
hCG (human chorionic gonadotropin) from the trophoblast acts like LH to maintain the corpus luteum, so progesterone and estrogen keep the endometrium from being shed. hCG appears in the mother's blood and urine about a week after fertilization — it's what pregnancy tests detect. By the 2nd–3rd month the placenta takes over hormone production and hCG levels fall.
3The placenta & membranes
The placenta forms from embryonic tissue (chorion — trophoblast-derived, with chorionic villi) and maternal tissue (decidua basalis of the endometrium). Maternal blood fills the intervillous spaces; fetal blood stays inside capillaries in the villi — the two bloods normally never mix. Nutrients, O₂, antibodies (IgG) and wastes are exchanged across the thin villus wall.
| Placental function | Details |
|---|---|
| Exchange | O₂ and nutrients to the fetus; CO₂ and wastes to the mother |
| Endocrine | hCG, then estrogen and progesterone (maintain pregnancy); human placental lactogen (prepares breasts, glucose-sparing in mother); relaxin; CRH (timing of birth) |
| Barrier (imperfect) | Blocks many microbes but alcohol, nicotine, many drugs and some viruses (rubella, HIV, Zika) cross — teratogens |
- Amnion
- Transparent sac filled with amniotic fluid — cushions the embryo, maintains constant temperature, allows free movement and prevents adhesions.
- Yolk sac
- Forms part of the gut; source of the earliest blood cells and primordial germ cells.
- Allantois
- Becomes part of the umbilical cord and the urinary bladder.
- Chorion
- Outermost membrane; forms the fetal part of the placenta.
- Umbilical cord
- Two umbilical arteries (carry deoxygenated blood to the placenta) and one umbilical vein (carries oxygenated blood to the fetus).
4Germ layers & organogenesis
During week 2 the inner cell mass becomes a bilaminar embryonic disc: epiblast and hypoblast. During gastrulation (week 3), epiblast cells migrate through the primitive streak to form three primary germ layers — the trilaminar embryo.
| Germ layer | Gives rise to |
|---|---|
| Ectoderm (outer) | Nervous system, epidermis (hair, nails, skin glands), lens and cornea, tooth enamel, adrenal medulla |
| Mesoderm (middle) | Muscle, bone and connective tissue, blood and blood vessels, heart, kidneys and gonads, dermis, serous membranes, notochord |
| Endoderm (inner) | Epithelial lining of the digestive and respiratory tracts, liver, pancreas, thyroid, parathyroid, thymus, urinary bladder lining |
Neurulation: the notochord (mesoderm) signals the overlying ectoderm to thicken into the neural plate, which folds into a neural groove and closes into the neural tube (→ brain and spinal cord) by about week 4. Neural crest cells migrate out to form sensory and autonomic neurons, Schwann cells, the adrenal medulla and melanocytes.
By the end of week 8, all major organ systems are in place — the embryo becomes a fetus. The embryonic period (organogenesis) is when the conceptus is most vulnerable to teratogens. The fetal period (weeks 9–38) is mainly growth and maturation.
5Fetal circulation
The fetal lungs and liver are nonfunctional, so oxygenated blood from the placenta takes shortcuts:
| Fetal structure | Bypasses | After birth becomes |
|---|---|---|
| Umbilical vein | — (carries O₂-rich blood to the fetus) | Ligamentum teres (round ligament of the liver) |
| Ductus venosus | Liver (umbilical vein → IVC) | Ligamentum venosum |
| Foramen ovale | Lungs (right atrium → left atrium) | Fossa ovalis |
| Ductus arteriosus | Lungs (pulmonary trunk → aorta) | Ligamentum arteriosum |
| Umbilical arteries | — (carry blood back to the placenta) | Medial umbilical ligaments |
At birth, the umbilical cord is clamped and the newborn takes its first breath: the lungs inflate, pulmonary resistance falls, more blood returns to the left atrium, and left atrial pressure exceeds right — pushing the foramen ovale's flap shut. The ductus arteriosus constricts within hours to days.
6Maternal changes
| System | Change |
|---|---|
| Reproductive | Uterus grows from fist-size to reach the xiphoid process by full term; breasts enlarge |
| Musculoskeletal | Relaxin loosens pelvic ligaments and pubic symphysis; posture shifts (lordosis, backache) |
| Cardiovascular | Blood volume ↑ 25–40%, cardiac output ↑ 20–40%; uterus can compress the IVC (lie on the left side) |
| Respiratory | Tidal volume ↑; shortness of breath late as the uterus pushes up the diaphragm |
| Urinary | ↑ GFR, frequent urination (bladder compressed) |
| Digestive | Morning sickness (early); heartburn and constipation (late) |
| Metabolic | ↑ BMR; human placental lactogen makes mother's tissues more insulin-resistant — gestational diabetes risk |
7Labor & lactation
Late in pregnancy, rising estrogen (from the placenta) makes the myometrium more responsive: it increases oxytocin receptors and gap junctions and counters progesterone's quieting effect. Weak, irregular Braxton Hicks contractions may occur. Fetal stress hormones and placental CRH help time birth.
- 1The baby's head pushes on the cervix, stretching it.
- 2Stretch receptors signal the hypothalamus → posterior pituitary releases oxytocin.
- 3Oxytocin (and prostaglandins it stimulates from the placenta) cause stronger uterine contractions.
- 4Stronger contractions push the baby harder against the cervix → more oxytocin — the cycle intensifies until delivery.
| Stage of labor | Events | Duration (first birth) |
|---|---|---|
| 1. Dilation | Cervix effaces (thins) and dilates to 10 cm; amnion ruptures (“water breaks”) | Longest — 6–12+ hours |
| 2. Expulsion | Full dilation → delivery of the infant; usually head first (vertex) | Up to ~2 hours |
| 3. Placental | Delivery of the placenta (afterbirth) ~15–30 min after the baby; uterine contractions compress vessels and limit bleeding | Up to ~30 min |
- Prolactin
- From the anterior pituitary — stimulates milk production. Suckling maintains its release. Placental estrogen and progesterone suppress milk production until after birth.
- Oxytocin
- From the posterior pituitary — triggers the let-down (milk ejection) reflex by contracting myoepithelial cells around alveoli; also helps the uterus contract back to size.
- Colostrum
- First secretion (first 2–3 days) — less fat, more protein, vitamin A and IgA antibodies, protecting the newborn's digestive tract.