Q&A: My MSAFP test came back positive and the numbers were 2.04, but I don't know what that means.

Positive results on an MSAFP (or maternal serum alpha-fetoprotein) blood test can be a cause for concern, but a positive test does not necessarily mean that your baby will have a birth defect.

Your MSAFP results are just one part of a more extensive screening test you will be given around 16 weeks of pregnancy. A blood sample is drawn and analyzed for four constituents, alpha-fetoprotein and these three hormones: human chorionic gonadotropin (hCG), estriol, and Inhibin. This so-called "quadruple screen" or "quad screen" aims to detect pregnancies at higher than random risk for two types of birth defects: Down syndrome and neural tube defects (such as spina bifida and the brain defect called anencephaly).

It is not clear from your question whether you had the triple screen or the quadruple screen. The detection rate for defects such as spina bifida, relying primarily on the MSAFP, is 80 percent to 90 percent and close to 100 percent for anencephaly. In the case of fetal uncomplicated Down syndrome, the MSAFP levels are low.

The figure you quoted was in the upper range and suggests that your result is in the approximately 95th percentile for MSAFP. While this could be normal, it calls for immediate attention in the form of a high-resolution ultrasound study of the fetus. A normal reading should largely set your mind at rest for a condition such as spina bifida or anencephaly.

Some obstetricians will offer an amniocentesis for prenatal studies of the amniotic fluid at the level you report. This fluid would be analyzed for the alpha-fetoprotein and a chemical (enzyme) called Acetylcholinesterase, which may leak from the nervous system if the fetus has an open spina bifida defect. Others who have excellent ultrasound experts available might only offer the amniocentesis if your levels for MSAFP approximated the 97th percentile.

The overwhelming majority of women with your results will not have a child with a birth defect. However, assuming the ultrasound report is normal, this level does indicate somewhat higher risks for late pregnancy complications, including elevated blood pressure and a low birth weight infant. Make sure to keep all of your prenatal appointments and talk to your healthcare provider about any concerns that you may have.