WOMEN: MULTIPLE PREGNANCIES
Normally, the average pregnancy consists of one single foetus. But about once in ninety cases, twins occur.
The rate at which triplets occur is 1 in 90 x 90 pregnancies, and quadruplets 1 in 90 x 90 x 90. This is the rate in Western lands. It is more common in Africa and Asia where a larger hereditary factor apparently plays a part.
There are two types of twins: "Binovular" twins occur when two separate eggs (or ova) are released and fertilized at the same time. Each foetus is a separate entity and is quite distinct from its fellow. "Uni-ovular" twins come from one single egg which has prematurely divided into two separate entities.
Twins are often diagnosed before their birth, but about 20 per cent are not recognized until the moment of delivery. Features which make a patient and the doctor suspect multiple births are mainly the size of the abdomen and womb which are larger than would be expected at any given time.
Sometimes two heads may be felt through the abdominal wall by the doctor's examining hands. Frequently, the only sure way is by X-ray or ultra-sound examination of the womb. Two separate heads and bodies can be clearly detected in this manner.
Generally, multiple births proceed to term normally. But there is an increased risk of certain complications taking place, so the mother may be requested to attend more frequently for her prenatal examinations. Some cases end in premature labour.