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.Continue Reading
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.Learn more about Diagnostics & Imaging