Gestation and Birth
High-Yield Summary
- Human gestation ≈ 280 days (40 weeks), 3 trimesters. Larger species generally: longer gestation, fewer offspring.
- 1st trimester: organogenesis — major organs form; embryo becomes a fetus.
- 2nd trimester: rapid growth, quickening (first felt fetal movement), face/fingers/toes become defined.
- 3rd trimester: accelerated brain development + maternal IgG transfer across placenta → passive immunity for the newborn.
- Birth sequence: cervical effacement → membrane rupture → contractions (prostaglandins + oxytocin) → delivery → afterbirth.
Key Terms
- Organogenesis
- Formation of the major organs, dominating the first trimester; marks embryo → fetus transition.
- Quickening
- The pregnant person's first felt fetal movement, typically in the second trimester.
- Passive immunity
- Protection the newborn gets from maternal IgG crossing the placenta (heaviest in 3rd trimester), before its own immune system matures.
- Effacement
- Softening and thinning of the cervix, the first step of birth.
The Three Trimesters
| Trimester | Key Event |
|---|---|
| First | Organogenesis; embryo becomes a fetus |
| Second | Rapid growth; quickening (fetal movement felt) |
| Third | Brain development accelerates; maternal IgG transfer (passive immunity) |
Stages of Birth
- 1Cervical effacement — cervix softens and thins.
- 2Membrane rupture — amniotic sac ruptures ("water breaking").
- 3Contractions — prostaglandins + oxytocin drive uterine contractions, pushing the fetus downward.
- 4Delivery — contractions culminate in birth.
- 5Afterbirth — placenta and umbilical cord expelled.
Common MCAT Trap
- Passive immunity (maternal IgG) transfer peaks in the 3rd trimester, NOT the 1st — premature infants may lack sufficient transferred antibodies.
- Organogenesis defines the 1st trimester, not general "growth" — rapid growth is the 2nd trimester's hallmark.
- Two separate hormones drive labor contractions: prostaglandins AND oxytocin — don't attribute it to just one.
Quick Recall
Which trimester is defined by organogenesis?
What antibody class transfers across the placenta, and when is transfer heaviest?
What are the two hormones that drive labor contractions?
What is quickening?