What is alpha fetoprotein?

Alpha fetoprotein (AFP) is a protein substance produced by the liver of the foetus, and binds the hormone, estradiol.

A small amount of this protein is excreted from the kidney of the foetus into the amniotic fluid, and then passes from the amniotic fluid into the mother's bloodstream.

The concentration of AFP in the mother's blood rises gradually into late pregnancy and can be easily measured by a blood test called the maternal-serum-alpha-fetoprotein test, or MSAFP. So, remember that there is always a certain amount in the mother's blood in normal pregnancies.

When is testing done?

The MSAFP is a routine screening blood test if certain foetal disorders are suspected. It is generally offered to women between the 15th and 20th week of pregnancy, and is most accurate when performed between 15 and 18 weeks' gestation.

What does this test screen for?

Abnormal MSAFP levels are associated with some genetic conditions, such as Down's syndrome, and certain birth defects such as spina bifida or intestinal strictures, growth retardation and some late-pregnancy complications.

As fetal genetic disorders are more common in women aged over 35, MSAFP is typically offered to pregnant women over this age, and to those with a relevant family medical history. Certain defects, on the other hand, such as those causing abnormalities within the developing brain/spinal cord, can occur in previously unaffected families.

The AFP test

Blood can be drawn for this test at the same time as other prenatal tests. Your GP or midwife will talk with you about the reasons for this test, and the procedure that will be suggested should there be abnormal findings, and seek your consent prior to testing.

A woman has the right to refuse this test if she chooses. If the test reveals abnormal values, a follow-up with an ultrasound and amniocentesis may be recommended for a more definitive diagnosis.

Women who have opted for amniocentesis do not need the MSAFP because the AFP test is included in amniocentesis screening.

The results

Normal levels and accuracy of the test are dependent on factors such as maternal age, race, weight and gestational age. A complete medical history must accompany the sample to allow accurate interpretation of the results by the specialist who will look at the results.

The information supplied will include such details as date of last menstrual period, estimated date of delivery, date of ultrasound, if the mother is an insulin-dependent diabetic, and if there is a family history of Down's syndrome, chromosomal abnormalities or neural tube defects.

MSAFP blood levels fall in three ranges: low, normal or elevated. An elevated MSAFP level (2 1/2 times above normal) is associated with open neural tube defects (like spina bifida) and central nervous system defects (anencephaly).

Elevated levels are also associated with twins and an open anterior abdominal wall defect of the foetus (where the front wall of the abdomen has not closed properly during development). Elevated levels may also play a role in late-pregnancy complications such as premature delivery, intrauterine growth retardation (not growing properly within the womb) and haemorrhage.

Abnormally low MSAFP levels are associated with Down's syndrome and other chromosomal abnormalities. However, a high or low result does not necessarily mean that the baby has any of those abnormalities.

Frustrating, isn't it? This, of course, is important to remember. Any actions that may be suggested, as a result of abnormal findings, will only be carried out when the patient herself has given permission.

Usually, a second opinion will always be added as a matter of course, if findings suggest a drastic action.

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.