The placenta and the hormones of pregnancy

An embryo that cannot eat or breathe

After implantation, the embryo lies buried in the wall of the uterus. It has no mouth that can eat and no lung that can breathe. All it needs must come from the blood of the mother, and all it makes as waste must go back to her. The organ that makes this possible is the placenta.

How the placenta forms

1. After implantation, finger like projections called chorionic villi appear on the trophoblast.
2. The villi are surrounded by the tissue of the uterus and by the blood of the mother.
3. The chorionic villi and the uterine tissue become interlocked. Together they form the placenta, a structural and working unit between the embryo and the body of the mother.
4. The placenta is joined to the embryo by the umbilical cord. The cord carries substances to and from the embryo.

The word "chorion" is the outer membrane that forms from the trophoblast. The villi belong to the embryo, not to the mother.

What the placenta does

• It supplies oxygen and nutrients to the embryo.
• It removes carbon dioxide and the waste made by the embryo.
• It lets the antibodies of the mother pass to the embryo, so a baby is born with some defence against germs.

The blood of the mother and the blood of the embryo do not mix. They flow on two sides of a thin barrier. Oxygen and food pass across it by diffusion and other transport. The umbilical cord has two arteries, which carry blood from the foetus to the placenta, and one vein, which carries blood from the placenta to the foetus.

The placenta as a gland

The placenta is also an endocrine tissue. It makes several hormones.

• hCG, human chorionic gonadotropin.
• hPL, human placental lactogen.
• Oestrogens and progestogens.

In the later phase of pregnancy the ovary also secretes relaxin. The hormones hCG, hPL and relaxin are made in a woman only during pregnancy.

In addition, the levels of oestrogens, progestogens, cortisol, prolactin and thyroxine rise many times in the blood of the mother. These changes support the growth of the foetus, the changes in the body of the mother and the keeping of the pregnancy.

This part is not in the NEET syllabus, but A-Level and IB ask it. The hCG keeps the corpus luteum working for the first weeks. The corpus luteum goes on making progesterone, so the endometrium is not shed. At about week 8 to week 10, the placenta takes over most of the work of making progesterone. Urine tests for pregnancy look for hCG, because only a pregnant woman makes it.

You should find that hCG is the hormone that peaks early, while oestrogen and hPL rise most near the end.

Try it in Python

The program prints the relative levels of the four hormones at five weeks of the model.

Practise with new numbers

Common mistakes

• Saying that the blood of the mother flows into the embryo. The two bloods stay apart. Substances cross the thin barrier.
• Saying that hCG is made by the pituitary. It is made by the placenta.
• Saying that the umbilical cord carries the blood of the mother. The vessels of the cord carry the blood of the foetus.
• Saying that hCG, hPL and relaxin are made in every woman. They are made only during pregnancy.
• Saying that the villi belong to the uterus. They grow out of the trophoblast of the embryo.

Remember these

• Chorionic villi appear on the trophoblast. They interlock with the uterine tissue to form the placenta. The umbilical cord joins the placenta to the embryo.
• The placenta gives oxygen and food, and removes carbon dioxide and wastes.
• Placental hormones: hCG, hPL, oestrogens, progestogens. The ovary makes relaxin in the later phase.
• hCG, hPL and relaxin are made only during pregnancy.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does the placenta form?

2. What does the umbilical cord do?

3. Name two substances that go from the mother to the embryo, and two that go the other way.

4. Name three hormones that the placenta makes.

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

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 28, Development and Inheritance.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 82.