Second trimester screening is a blood test done a little later in pregnancy, usually between 15 and 20 weeks (some laboratories accept samples up to 22 weeks). It estimates the chance that the baby has certain chromosome conditions or a problem with the development of the spine or brain. In India, it is most often called the “quadruple marker” or “quad marker” test. An older version with three markers, the “triple marker” test, is still used in some places.

Many families have this test because they first saw a doctor after 14 weeks and missed the window for first trimester screening. Others have it because first trimester screening or Non-Invasive Prenatal Testing was not available or affordable.

Second trimester screening looks mainly for three conditions:
• Down syndrome (trisomy 21) An extra copy of chromosome 21. It causes learning disability of varying degree, and some babies have heart or bowel differences.
• Edwards syndrome (trisomy 18) An extra copy of chromosome 18. It causes serious problems with the heart and other organs, and many pregnancies do not continue to birth.
• Open neural tube defects Conditions where the spine or skull does not close fully in early pregnancy. The two main types are open spina bifida (an opening in the spine) and anencephaly (where much of the brain and skull do not form). This is something first trimester screening does not check for.

Chromosome conditions usually happen by chance when the egg or sperm is formed, not because of anything the parents did. The chance of Down syndrome rises with the mother’s age, but it can happen in any pregnancy. This is why guidelines from the American College of Obstetricians and Gynecologists recommend offering screening to every pregnant person, whatever their age.
Second trimester screening is a screening test, not a diagnosis. It gives a chance, not a yes or no answer. It does not check for single-gene conditions such as beta-thalassemia, sickle cell disease, or spinal muscular atrophy. These need separate carrier screening of the parents.