Maternal serum-alpha-fetoprotein (MSAFP)

Neural tube defects are detected in approximately one out of every 1,000 to 1,500 pregnancies. A positive test result will occur in one to two per cent of all women undergoing AFP testing.

The AFP test identifies 80 to 90 per cent of foetuses with spina bifida, anencephaly or abnormality of the abdominal wall. For pregnant women age 35, the risk of Down's syndrome is one in every 270 pregnancies, increasing in risk with maternal age.

Women who are tested in their second trimester may be faced with the dilemma of deciding whether to carry their babies to term or to terminate the pregnancy.

It is important to recognise that not all suspicious results indicate a problem with the foetus, just as normal results do not definitively rule out any potential problems.

False-positive results

False-positive results are a concern with MSAFP screening. False-positive readings from elevated levels of AFP can be due to twins, threatened abortion, inaccurate dates, low-birth-weight infants, a placental defect (large or malpositioned) or a sample contaminated with blood.

Abnormally low readings may result from incorrect dating, a female foetus or diabetes. I have, in fact, seen more than one patient present with abnormal investigation results who have gone on, even after counselling, and when heart-searching decisions were made, to deliver normal babies.

Optional testing

The triple screen, which includes three markers - MSAFP, human Chorionic Gonadotropin (HCG) and oestriol - is a more accurate test than the MSAFP.

It detects chromosomal anomalies, including Down's syndrome. Ideally, this test is performed between 16 to 18 weeks from the last menstrual period. It improves the detection of chromosomal anomalies by 60 per cent among women aged 35 and over.

Quadruple test, which also measures levels of the hormone, inhibin A, and the latest test, the Penta Screen, which adds in measurement of Invasive Trophoblast Antigen, have also been developed.

If the triple, quadruple or Penta screens are available in your area, they will be requested in preference to the AFP alone.

These tests can help pregnant women avoid unnecessary amniocentesis, and having to face the potentially distressing decisions which result from any abnormal findings.