First three months of pregnancy
The term embryo is used to describe the developing baby until the end of the eighth week. Two membranes form around the developing embryo and on one side fingers probe into the lining of the uterine wall to become the placenta. The placenta is a spongy structure that is responsible for the exchange of nutrients, gases and wastes between the mother’s blood and that of the embryo. The mother’s blood in the uterine wall is separate from the embryo’s blood in the placenta. The second membrane forms around the developing baby, called the amniotic sac. The placenta is attached to the developing baby by the umbilical cord.
By the end of the fourth week the embryo is about a quarter of a centimetre long. The brain, spinal cord, nervous system and throat have begun to develop.
At the end of the eighth week the embryo is 3cm long, with the face, legs, ears and nose visible. It is during these first eight weeks that morning sickness is most common and some women suffer nausea. After the eighth week the developing baby is called a foetus.
Fourth to sixth months of pregnancy
By the end of the twelfth week the foetus is about 8cm long. Morning sickness tends to lessen or disappear. As the foetus develops, the woman’s abdomen stretches and protrudes. In the fourth month a woman might feel the foetus move for the first time.
During the fourth, fifth and sixth months the woman will increase in weight by approximately 5kg with about 1kg of that being the baby. By the sixteenth week the entire body structure is formed, sucking motions might begin and the sex of the foetus can be determined. The foetus would be approximately 14cm long and weigh approximately 150 grams.
At the end of week 20 hair is visible, the skin is pink and the heartbeat can be heard with a stethoscope. The foetus would be about 20cm long and weigh 300 grams. By week 24 the eyelids are open, the skin red and wrinkled, the foetus would be about 25-30cm long and weigh around 500-600 grams.
Seventh to ninth month of pregnancy
During the last three months of pregnancy the woman’s size is increasing rapidly and foetal movement is common. Weight gain during this time is about 4kg, but this will vary significantly depending on the race and size of the mother and the sex of the baby.
By the end of week 28 the head and body are more proportionate. Often by now the foetus has moved so the head is down towards the cervix. The foetus will be about 28cm long and weigh approximately 1kg. A baby born at this stage will often survive if given intensive care.
At the end of week 32 the foetus will be about 35-40cm long and weigh up to 1.5kg. By the time the baby is ready to be born at 38 weeks the foetus will be about 45cm long and weigh around 3kg.
Stages of birth
Late in pregnancy the foetus generally positions its head deep in the pelvic cavity. The birth process is also called labour and for convenience of explaining the sequence of events it is divided into three stages.
First stage of labour
The first stage of labour often begins with the woman having contractions of the uterus. It is generally the longest stage and, particularly for the first birth, can last from 10-16 hours. At first the contractions are short and mild and occur at intervals of 10-20 minutes. As the birth continues the contractions come more frequently every 3-5 minutes, last longer and are more intense. Early in this stage the cervix becomes dilated up to 4cm, then eventually up to 10cm. Also during this phase the amniotic sac, which surrounds the baby, will rupture. This is often called ‘the waters breaking’.
Second stage of labour
The second stage begins when the cervix is completely dilated and the baby moves further down into the vagina, or birth canal. Usually the baby is born head first. The second stage can last from half an hour to two hours, although this varies, and is complete when the baby is born. During this time the woman can actively push to help the baby out.
Third stage of labour
Contractions continue after the baby is born. The placenta separates from the uterine wall and comes out of the vagina generally within an hour of the birth. The placenta is also called the afterbirth.