PLACENTA

Throughout pregnancy, the placenta is the vital link between mother and fetus. Both the feeding of the baby and the elimination of its waste products occur through this organ, which is expelled after the baby’s birth.

FIGURE: THE ANATOMY OF THE PLACENTA

The placenta or afterbirth forms when a specialized part, of the fertilized egg, called the trophoblast, embeds in the wall of the mother’s uterus (womb). By the 12th week of pregnancy the placenta is a separate organ; at the time of the baby’s birth it weighs approximately 500g (a little over a pound) and is dark red, spongy and disc-shaped. Two layers of cells keep the circulation of the fetal blood in the placenta separate from the mother's blood, but many substances can pass from mother to baby.

FUNCTION

All the food and oxygen the fetus needs it receives from the mother, and it is able to eliminate any waste products back into her. This vital exchange function is carried out by the placenta, to which the fetus is attached by the umbilical cord. Carbon dioxide, waste products and hormones pass from the fetus to the mother; oxygen, nutrients (simple carbohydrates, fats and amino acids) and hormones are transferred in the opposite direction.

The placenta also acts as a barrier to protect the fetus from potentially harmful substances, although many drugs can cross the placenta and harm the fetus.

Some of the mother's antibodies, too, cross the placenta.

Finally, the placenta produces several hormones, some of which prevent the woman releasing more eggs or having more periods while she is pregnant. They also encourage breast development in preparation for breast feeding, and the laying down of fat on the thighs, abdomen and buttocks as a future energy store. Other hormones also stimulate the growth of the womb and probably inhibit it from contracting before labour starts. There is also evidence to suggest that the amounts of these hormones released by the placenta may be an important factor in determining when labour starts.

MONITORING THE PLACENTA

The absolute proof of a healthy placenta is the birth of a healthy baby. However, the efficiency of the placenta is often checked during pregnancy by measuring the amounts of hormones it releases into the mother's blood. It is assumed that if the placenta is producing enough hormones it is also working well in all other respects. If the hormone levels fall and it is suspected that the fetus is not receiving adequate nourishment in the uterus, the baby may be delivered early.

DELIVERING THE PLACENTA

The placenta is normally delivered a few minutes after the baby's birth. The mother is given an injection of a drug called Syntometrine which makes the uterus contract almost into a tight ball. The large maternal blood vessels to the placental site are squeezed shut and the placenta is sheared off the wall of the uterus. The midwife then delivers the placenta through the vagina by pulling very gently on the umbilical cord and examines it to ensure that it is complete.

PROBLEMS

Apart from the placenta occasionally being unhealthy it can cause other problems. In rare cases it becomes partly or completely detached from the uterine wall before the baby is born. This causes pain and bleeding and must be treated urgently in hospital to save both the mother's and the baby's lives.

The placenta can also completely cover the neck of the womb, blocking the baby's passage through the birth canal. When this condition, called placenta praevia, occurs delivery is by Cesarean.

Sometimes a small piece of placenta remains stuck in the uterus and does not come away in the normal manner at delivery. This can cause severe blood loss, or secondary post-partum hemorrhage, several days after delivery. If an ultrasound scan reveals a piece of retained placenta, it is necessary to perform an evacuation of the uterus. This is a very simple procedure and is carried out under general anesthetic.