How the placenta and uterus develop


The placenta begins to develop when the egg (the ovum) attaches itself to the lining of the uterus a few days after ovulation. By the end of the 16th week it has reached its full size and it acts as the lungs, liver, intestines, kidneys and source of nutrition for the fetus. It must function normally for the baby to grow and develop.

Food and oxygen travel from the placenta to the fetus via the umbilical cord. The veins in the umbilical cord carry the baby’s blood to be oxidised in the placenta and at the same time transfer any waste substances to the mother’s circulation. The placenta separates the circulations of mother and baby. The entire metabolism takes place through a thin membrane. A fully developed placenta weighs about 500-600 grams and it is the shape of a flat disc. Placenta is a Latin word meaning a cake.


Examination of the fetus

The health of the fetus in the womb can also be examined, though participation in all these examinations is voluntary. Certain heightened risks of disease or can be determined by means of echography (ultrasound). Many local authorities offer all women over the age of 37 a chromosome study, because the older the mother is, the higher the risk of a chromosome defect in the baby is. Many places also offer serum screening by means of a blood test.

Results indicating a higher risk do not always mean that the baby has a disease or deformity. Nor are all deformities detected in screening. The older the mother is, the more reliable the test is.

Women whose results show that they are at risk are offered a closer examination that is conducted using a sample of the amniotic fluid or placenta. This test must be done before the 20th week of pregnancy, because any decisions to abort on grounds of fetal deformity must be made before then. In any case the mother has the right to refuse an abortion whatever the risks. The decision may be easier to make if she has had a chance to think over the various alternatives before going for the tests.


Before the start of pregnancy the uterus weighs only 50-70 grams and is 7 -9 cm long. By the end of pregnancy it weighs nearly one kilo and has a volume of about 5 lit- res. During pregnancy it grows with the baby. Between weeks 13 to 16 it grows upwards and begins to form a visible bulge. By the 16th week it has risen half way between the pelvis and the navel, by the 24th week it is up to the navel and by the 36th week it is up to the diaphragm. Normal uterus growth indicates that the placenta is functioning properly, whereas accelerated growth is often a sign of a multiple pregnancy.


The uterus gradually prepares for the delivery. During the final weeks you will begin to feel contractions, during which your tummy goes hard. To begin with the contractions last only a few seconds, but towards the end they may last as long as half a minute. What they are doing is getting the cervix ready for the delivery. During the last 4 weeks the cervix may dilate about a centimetre, or if you have had a baby before, as much as four centimetres. The top of the uterus (the fundus) usually drops a couple of weeks before the birth and the womb becomes rounded. At the same time the baby in most cases turns head-down ready for delivery.