A Summary of Foetal Development in the 3 Trimesters

First Trimester

0-4 Weeks

Once the fertilized egg embeds itself into the lining of the uterus, the cells begins to divide rapidly. The formation of the foetus’s body parts commences. The embryo (the egg) can be seen with the naked eye by the 4th week. The head and tail emerges together with an attached yolk sac which provides food for the embryo until the placenta is fully developed.

5-8 Weeks

The embryo is about 2mm long by the 5th week. The heart can now be seen beating and circulating blood via an ultrasound scan. The head is almost taking its final shape: the spinal cord and the brain are growing swiftly. Towards the end of this month, the embryo will grown to the size of an one-dollar coin and the first minute movements of the spine become evident.

9-12 Weeks

By the 11th week, the foetus (as it is called now) resembles a small human being and is about 5.5cm long. The foetus’s muscles have begun to develop. Its movements are gradually becoming stronger. It is also starting to respond to mother’s strong body movements. By the end of this month, it is about 7.5cm long and weighs around 14g, including that all the major organs are completely formed.

Second Trimester

13-16 Weeks

The role of nourishing the foetus is now taken over by the fully developed placenta. The foetus is now 16cm long. It is covered in a fine downy hair (lanugo) and has fingernails and toenails. The foetus, placenta and waters fill up the whole of the uterus. The foetus starts to swallow and pass urine. It begins to experience the food that mother eat.

17-20 Weeks

The mother starts to feel her baby squirming inside as the foetus stretches and exercises its limbs. It is now 25cm in length and weighs 340g. The foetus’s eyes are still tightly closed whilst its hair has started to grow and its teeth are starting to form. By the end of this month, the foetus will be half of its length at birth.

21-24 Weeks

The foetus now has clear-cut periods of sleep and wake. Besides the mother’s heartbeat and blood circulation, sudden loud noises and music or other sounds can be heard by the foetus and it begins to react to these external sounds. If the foetus were to be delivered prematurely, special care is needed if it is to have a chance of survival. By the end of this month, it is approximately 33cm long. It can now open its eyes and as its eyesight improves, it is aware of light changes outside the womb.

25-28 Weeks

The mother will be able to feel the foetus more: its hiccups or its kicks when it’s getting much stronger. It can be felt by placing a hand on the mother’s belly. The foetus is swathed in a fatty substance (vernix) which protects its skin from the amniotic fluid. It is now about 37cm long.

Third Trimester

29-32 Weeks

The foetus is now fully developed. It is ready to take its first breath as its lungs starts to mature. It could be sucking its thumb now. The foetus is now approximately 38cm and weighs about 1.6kg.

33-36 Weeks

The foetus is now passing urine, swallowing fluid and blinking its eyes. To regulate its body temperature better after birth, it still needs to put on more weight. As it now fills up the whole womb and is unable to twist and turn more freely, its position in the uterus is final. As it stretches its limbs, you can now tell whether the protrusion on the mother’s belly is a hand or foot.

37-40 Weeks

Except perhaps for the shoulders, the lanugo has usually vanished by now. The foetus is tucked snugly into a ball, looking pink and round. The baby is ready to emerge from the womb once its head descends into the mother’s pelvis (engages). When this happens, the mother may be able to breath more easily.