The quadruple screen test is typically performed when prenatal care begins in the second trimester, or if a first-trimester screening (including blood tests and ultrasound examination) was not conducted. This test measures the levels of four specific markers in the mother's blood to assess the risk of certain chromosomal abnormalities and neural tube defects. The markers measured are:

* Alpha-fetoprotein (AFP): A protein produced by the developing baby.
* Human Chorionic Gonadotropin (hCG): A hormone produced by the placenta.
* Estriol (E3): A hormone produced by the placenta and the baby's liver.
* Inhibin A: Another hormone produced by the placenta.