Parturition and lactation

The end of pregnancy

The pregnancy of a woman lasts about nine months. At the end, the muscle of the uterus contracts strongly, and the baby is pushed out. The delivery of the baby is parturition, or childbirth.

What starts parturition

Parturition is controlled by nerves and hormones together. The steps are these.

1. The fully developed foetus and the placenta send signals.
2. The signals cause mild contractions of the uterus. These are called the foetal ejection reflex.
3. The reflex makes the pituitary gland of the mother release the hormone oxytocin.
4. Oxytocin acts on the muscle of the uterus and makes the contractions stronger.
5. The stronger contractions cause the release of more oxytocin.
6. This chain goes on, and the contractions become stronger and stronger. At last the baby is pushed out through the birth canal. Soon after, the placenta is also expelled.

The ovary, and in some books the placenta, also release relaxin in the later weeks. It makes the ligaments of the pelvis looser.

A loop that strengthens itself

In the loop above, a result of the process makes the process stronger. The stronger contractions cause more oxytocin to be released, and the extra oxytocin makes them stronger still. This is positive feedback. Most controls of the body are negative feedback, in which the result turns the process down. A positive loop is rare. It is used when a process must finish quickly, such as the birth of a baby. It ends when the cause is taken away, that is, when the baby has left the uterus.

This part is not in the NEET syllabus, but A-Level and IB ask it. Doctors describe labour in three stages: the opening of the cervix, the birth of the baby, and the delivery of the placenta, which is the afterbirth. After birth, the cord is tied and cut.

You should find that a larger gain or a stronger start makes birth come sooner. A gain of 1 would never reach the limit, because nothing would strengthen the contractions.

Try it in Python

The program counts the rounds of contractions needed in a model of positive feedback.

Lactation

The mammary glands change during pregnancy. By its end they have grown and begin to make milk. The making of milk is lactation. It lets the mother feed the new born baby.

• The milk made in the first days is the colostrum. It holds several antibodies that the new born needs to resist germs.
• Doctors advise breast feeding in the early life of the baby. The World Health Organization advises breast milk alone for the first six months.

Two hormones work in lactation. Prolactin from the anterior pituitary makes the alveoli produce milk. Oxytocin from the posterior pituitary makes the muscle cells around the alveoli contract, so the milk is pushed into the ducts. The sucking of the baby at the nipple sends nerve signals to the brain. They cause the release of both hormones, so the more the baby sucks, the more milk is made.

Practise with new numbers

Common mistakes

• Saying that oxytocin is made by the placenta. The mother's pituitary gland releases it.
• Calling parturition a negative feedback. The contractions make more oxytocin come out, which is positive feedback.
• Saying that the foetus has no part in starting birth. The signals come from the fully developed foetus and the placenta.
• Saying that colostrum is poor milk. It holds many antibodies and is very important for the baby.
• Mixing up the two hormones of lactation. Prolactin makes the milk. Oxytocin pushes it out.

Remember these

• Parturition: signals from the foetus and the placenta, then the foetal ejection reflex, oxytocin, stronger contractions, more oxytocin, birth, and then the placenta is expelled.
• The loop is positive feedback.
• Lactation: the first milk is the colostrum, with antibodies. Prolactin makes milk, and oxytocin ejects it.

Check yourself

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

1. What is parturition?

2. What is the foetal ejection reflex?

3. What does oxytocin do in parturition?

4. What is the colostrum, and why is it needed?

5. Which hormone makes the milk, and which hormone pushes it out?

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.
• World Health Organization (2023). Infant and young child feeding, fact sheet.