Pregnancy, Placenta, Birth and Lactation: Quick Revision

A brief note on Pregnancy, Placenta, Birth and Lactation. Read it before a test. Try each question at the end before you open its answer.

The placenta and its hormones

• Formation: chorionic villi appear on the trophoblast after implantation and interlock with the uterine tissue. The result is the placenta, joined to the embryo by the umbilical cord.
• Work: it gives oxygen and nutrients, and removes carbon dioxide and wastes. The two bloods do not mix.
• Hormones: hCG, hPL, oestrogens and progestogens from the placenta. Relaxin from the ovary in the later phase. hCG, hPL and relaxin are made only in pregnancy.

Development

• Inner cell mass: stem cells that form the three germ layers, ectoderm, mesoderm and endoderm.
• Timeline: heart after 1 month. Limbs and digits by 2 months. Organ systems by 12 weeks. First movements and hair in the fifth month. Fine hair and eyelashes by 24 weeks. Fully developed at 9 months.
• Length: about 9 months, 280 days or 40 weeks, from the first day of the last menstruation.

Birth and milk

• Parturition: signals from the fully developed foetus and the placenta start mild contractions, the foetal ejection reflex. The pituitary releases oxytocin, which strengthens the contractions. This is a positive feedback. The baby is born, and then the placenta is expelled.
• Lactation: the mammary glands make milk towards the end of pregnancy. The first milk is the colostrum, rich in antibodies. Prolactin makes milk, and oxytocin pushes it out.

A worked example

A baby is born at 36 weeks. How many days early is that, if a full pregnancy lasts 280 days?

1. Days at birth: $36 \times 7 = 252$.
2. Days early: $280 - 252 = 28$.

Practice with new numbers

A program to try

The program gives the length of a full pregnancy in weeks and in months.

Mistakes to avoid

• Saying that the blood of the mother flows into the foetus. The two bloods stay apart.
• Saying that hCG comes from the pituitary. The placenta makes it.
• Saying that the placenta comes from the inner cell mass. The villi come from the trophoblast.
• Mixing prolactin and oxytocin. Prolactin makes milk. Oxytocin pushes it out and strengthens the uterus.

Check yourself

1. Which hormones are made in a woman only during pregnancy?

2. What is the foetal ejection reflex?

3. Why is the control of birth a positive feedback?

4. Why is the colostrum important?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 28.
• World Health Organization (2023). Infant and young child feeding, fact sheet.