Screening of the Pregnant Woman

In order to identify women with an increased risk of certain problems, including having a baby with a brain or spinal cord defect (neural tube defect), Down syndrome, other chromosomal abnormalities, or some rarer genetic disorders, blood levels of certain substances (called markers) can be measured. There are no risks associated with these blood tests for the fetus. By determining the woman’s individual risk of having an abnormal baby, they can help the couple better understand whether invasive prenatal genetic testing is worth the risk.

As part of routine prenatal care, doctors measure markers for chromosomal abnormalities. There are, however, some couples who decide that they do not wish to have any testing done. Other couples, such as those who are at a high risk for certain disorders, may skip these blood tests and proceed directly to invasive prenatal genetic testing (such as chorionic villus sampling or amniocentesis) instead of screening blood. In the event that a woman decides to undergo chorionic villus sampling, doctors usually advise her to have a blood test that will measure the level of a marker called alpha-fetoprotein (a protein produced by the growing fetus) along with the chorionic villus sampling. It is possible to determine the risk of birth defects of the brain or spinal cord (neural tube defects), such as spina bifida, by measuring the alpha-fetoprotein level in a pregnant woman. It is not possible to obtain this information from chorionic villus sampling.

Pregnancy markers are usually assessed between the 10th and 13th week of pregnancy (1st-trimester screening). It is also possible to measure other markers at 16 to 18 weeks of pregnancy as part of the second-trimester screening.