Inhibin A is a protein with two dimer subunits. Measured in females, at 16-18 gestational weeks, as part of a prenatal quad screen, increased levels (along with increased beta-hCG, decreased AFP, and a decreased E3) may suggest the presence of a fetus with Down syndrome. Abnormal results would suggest follow-up with definitive testing. Inhibin A has also been used as a marker for ovarian cancer.

In females, Inhibin is made by granulosa cells in the ovary and in males by Sertoli cells in the testes.