• Women who have fetal abnormalities show up on ultrasound or through other tests.

Some of these other blood tests include maternal alpha fetal protein, estriol and human chorionic gonadotropin. A new technique is being developed which allows a computer-generated risk to be calculated from the mother's age, pregnancy dates, weight, the results of the above blood tests, and diabetic testing. If the calculated risk is as high as that of a woman 35 or over, amniocentesis will then be offered to a younger woman.

• Women who have already given birth to one child with genetic defects or neural tubal defects.

Remember new evidence shows that taking multivitamins containing folic acid will prevent neural tube defects (but not genetic defects).

• Women with a personal or family history of inherited defects in cell metabolism and molecular disorders.

Over 200 different problems of this type can be diagnosed through amniocentesis.

Medical Reasons

Amniocentesis for medical reasons is done much later in pregnancy.

• Women with diabetes and other chronic illnesses.

In these women, amniocentesis is used in the last months of pregnancy to determine the maturity of the baby's lungs. This information is then used to decide the safest time to deliver the baby.

• Women who are RH negative and have formed antibodies to their babies.

This problem is uncommon now since it is totally preventable. But if for some reason antibodies have formed, then amniocentesis is used to tell how severely the baby is affected and whether the baby will require a transfusion while still in the womb.

Who Should Not Have Amniocentesis

Any woman who, for political or religious reasons, does not believe in therapeutic abortion for genetic reasons. However, some women may wish to have an amniocentesis done in order to prepare in advance for the birth of a handicapped child.

Amniocentesis should not be done for such reasons as to assess the damage caused by X-rays performed during pregnancy, or to check for damage from drugs taken by the mother (unless they are known to cause problems) or solely to check for the sex of the baby.

What Amniocentesis Can't Tell You

Amniocentesis can detect hundreds of different types of inherited abnormalities. Most of these are rare. Some abnormalities are not detectable through amniocentesis. Keep in mind that at present, the risk having a child with abnormalities with no testing varies between one and three percent.

A new blood test called alpha plus or triple screening may help you decide whether you are at high risk for a child with Down's Syndrome or neural tube defects. If the results show a woman is at low risk for these problems, she may want to reconsider the necessity of amniocentesis. This test enables a woman to get a reasonable estimate of her chance of having an abnormal baby without amniocentesis. (See the next chapter for more details).

As mentioned before, you can lower your risk of neural tube defects through folic acid supplements during pregnancy.

What Are The Risks

One baby in 200 is miscarried as a result of the procedure itself. If you have a tendency to miscarry, this fact must be weighed against the possible benefits of the procedure. Other side effects, such as puncture of the placenta, the baby, or the mother's bladder and infection of the amniotic fluid, are rare.