Foetal development takes a total of 9 months, so in obstetrics (the care of women during pregnancy and childbirth) this period of time is split into 3 calendar months. These are known in order as the first trimester, the second trimester and the third trimester.

The first trimester is a critical period of growth for the embryo. The heart is beginning to beat now and the first signs of breathing are evident due to early lung formation. There are first signs of bone cells formation, so the body features become apparent. The skin also has a translucent appearance and at this stage, the embryo can be harmed by radiation, microbes and drugs.

In the second trimester, the foetus’s skeleton is forming and the skin is becoming less transparent so it is beginning to take on a more human-looking form. The eyes are developing so now the eyelids can blink. Hair is beginning to grow and the foetus can even swallow. Blood vessels develop and by now most of the organs are fully formed.

In the third trimester, the foetus’s lungs start getting ready for breathing. The body begins to grow and fat deposits are stored in preparation for the birth. The lungs are more fully developed by now and brain development is rapid. The foetus can suck its thumb. The embryo is better able to withstand radiation, microbes and drugs owing to the protection of its mothers antibodies which it now receives.