From the inner cell mass to the foetus

Most of this lesson is not in the NEET syllabus, which stops at the placenta and at birth, but the boards, A-Level and IB ask it.

From a ball of cells to a body

You met the inner cell mass of the blastocyst in the last course. Its cells are stem cells. A stem cell has the power to give rise to every kind of tissue and organ.

After implantation, the inner cell mass changes into three layers of cells, the germ layers.

• The ectoderm is the outer layer.
• The mesoderm is the middle layer.
• The endoderm is the inner layer.

Every organ of the body is made from one of these three layers or from a mixture of them.

The foetus in its sac

The embryo grows inside a thin bag, the amnion. The bag is filled with amniotic fluid. The fluid cushions the foetus against knocks, keeps its temperature steady and lets it move. NCERT uses the word amniocentesis for the test in which a little of this fluid is taken out.

The word embryo is used for the early stages. From about the end of the second month the developing baby is called a foetus.

Growth month by month

The average length of human pregnancy is about nine months, the gestation period. NCERT lists these steps.

• After one month the heart is formed. The doctor can hear its sound with a stethoscope.
• By the end of the second month, the limbs and the digits have developed.
• By the end of 12 weeks, called the first trimester, most major organ systems are formed. The limbs and the external genital organs are well developed.
• During the fifth month, the first movements of the foetus are felt and hair appears on the head.
• By the end of 24 weeks, the body is covered with fine hair, the eyelids are separate, and the eyelashes are formed.
• By the end of nine months, the foetus is fully developed and ready for birth.

A trimester is a third of the pregnancy, about three months. Clinical books end the first trimester at week 13 and the second at week 27. NCERT writes 12 weeks and 24 weeks, so use its numbers in an Indian exam and the clinical numbers elsewhere.

The expected date of birth

Doctors count the weeks from the first day of the last menstruation, not from fertilisation. Fertilisation takes place about two weeks later. A full pregnancy lasts about 280 days, which is 40 weeks, counted from the first day of the last menstruation.

So the expected date of birth is the date of the last menstruation plus 280 days. A quick rule gives the same date: add 7 days, take away 3 months, and add one year. Only about 1 baby in 20 is born exactly on that date, and most are born within two weeks of it.

You should find that each trimester has three of the nine months, and that the weeks at the end of the month grow by a little more than four each time.

Try it in Python

The program works out the expected date of birth and the weeks of the pregnancy so far.

Practise with new numbers

Common mistakes

• Counting the weeks from fertilisation. Doctors count from the first day of the last menstruation.
• Mixing up the germ layers. The brain and the skin come from the ectoderm. The muscles and the heart come from the mesoderm. The lining of the gut comes from the endoderm.
• Saying that the inner cell mass makes the placenta. The placenta comes from the trophoblast and the tissue of the uterus. The embryo comes from the inner cell mass.
• Saying that the heart forms at nine months. It forms at the end of the first month.
• Mixing up the trimesters. Each of them is about three months.

Remember these

• The inner cell mass has stem cells. It forms three germ layers: ectoderm, mesoderm and endoderm.
• Heart after one month. Limbs and digits by the end of two months. Organ systems by 12 weeks. First movements and hair in the fifth month. Fine hair and eyelashes by 24 weeks.
• Pregnancy: about nine months, or 280 days, or 40 weeks, from the first day of the last menstruation.

Check yourself

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

1. Name the three germ layers, and give one organ that each of them forms.

2. What is a stem cell?

3. When does the heart form, and when do the limbs and the digits develop?

4. When are the first movements of the foetus felt?

5. A woman's last menstruation began on the 1st of January. About when is the baby expected?

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.
• American College of Obstetricians and Gynecologists (2017). Committee Opinion 700, Methods for estimating the due date.