Week 18

week_18At this point in the second trimester, you’re probably familiar with screening tests to help determine your baby’s risk for physical abnormalities or congenital disorders, some of which can be inherited. Between the 15th and 20th weeks of pregnancy, your physician may recommend another test: the quad screen.

Most birth defects develop during the first three months of pregnancy. Some of the most common birth defects discovered using screening tests include abdominal wall defects, heart defects or Down syndrome. Screening tests are offered to all pregnant women beginning in the first trimester, and women identified as being at increased risk for having a baby with a birth defect typically are offered diagnostic testing earlier.

During the first trimester, you may have been screened for Down syndrome and trisomy 18—an extra chromosome that can cause mental retardation—via blood tests and an ultrasound exam. Blood tests are used to measure the level of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG) in the blood, while ultrasound measures the thickness of the back of the fetus’ neck, where an increase can indicate problems.

Four Worth Finding

The quad screen is a blood test that takes only minutes to perform and could yield valuable information about your baby’s future health. The screen looks for four substances: alpha-fetoprotein (AFP), hCG, estriol and inhibin-A. All pregnant women are generally offered the quad screen, but it’s recommended for those 35 years or older who have a family history of birth defects, have used potentially harmful medications or drugs while pregnant, have diabetes and use insulin, have had a viral infection during pregnancy or have been exposed to high levels of radiation.

The quad screen detects high and low levels of AFP, abnormal levels of hCG and estriol and high levels of inhibin-A, which are combined with a woman’s age and ethnicity to assess her baby’s risk for genetic disorders. High levels of AFP may indicate a birth defect such as spina bifida, while low AFP levels and high hCG and estriol measurements could indicate Down syndrome or trisomy 18. Abnormal test results may warrant additional testing or amniocentesis to confirm a diagnosis.


  • Having abnormal results on a quad screening doesn’t mean your baby will have a birth defect. High levels of alpha-fetoprotein, for example, are most commonly caused by inaccurate dating of conception rather than a disorder or abnormality.
  • Why decide to have additional testing after a quad screening with abnormal results? Confirmation of a diagnosis may open the door to possible interventions, such as fetal surgery for spina bifida, or allow you to begin planning for a special needs child.