How do you read an abnormal alpha-fetoprotein number chart?

A chart of alpha-fetoprotein test results shows the probability that an abnormal finding indicates the fetus has trisomy 21 or spina bifida, says OB-GYN Ultrasound Online. There is some overlap in test results between these two groups and test results that indicate a healthy fetus. The doctor uses the chart to estimate the probability the fetus is affected and then uses other test results to make a final diagnosis.

Between 15 and 20 weeks of gestation, pregnant women carrying fetuses affected by trisomy 21 have lower than normal levels of alpha-fetoprotein in their blood, and those carrying fetuses affected by spina bifida have higher than normal levels of alpha-fetoprotein in their blood, says OB-GYN Ultrasound Online. The alpha-fetoprotein test results are converted into a measure called the multiple of the median, which takes into account the exact gestational age, mother's age, mother's weight, mother's ethnicity and diabetic status. A multiple of the median of one is considered to indicate the highest probability of having a healthy fetus.

The triple-screen test is a simple blood test performed on women between 15 and 20 weeks of gestation, says the American Pregnancy Association. The mother's blood is tested for alpha-fetoprotein, human chorionic gonadotropin and estriol. If the results of all three tests suggest the fetus is not healthy, further testing, such as an ultrasound or amniocentesis, is performed.