Second Trimester Screening, sometimes referrred to as triple screen or quad screen, is a blood test that is used to identify pregnancies at increased risk for certain birth defects, including neural tube defects, and two chromosomal disorders: Down syndrome and Trisomy 18. The results from this testing are not diagnostic. Second trimester screening is done between 15 and 20 weeks gestaion. If a patient has completed first trimester screening, a complete second trimester screen is not recommended; although a maternal serum AFP level should be drawn.