What Is Inhibin a?

Inhibin A is a hormone produced by a woman's body during pregnancy as well as during the menstrual cycle. The placenta — the tissue that nurtures a developing baby — makes inhibin A, which is detectable in an expectant mother's bloodstream. Usually, this hormone is discussed in relation to a pregnancy screen called the quad screening test. When inhibin A levels are higher than expected, this could mean the developing baby has Down syndrome or another abnormality. In addition, the ovaries produce this hormone as part of the menstrual cycle, and high levels are sometimes associated with tumor development.

When a baby is growing in his mother's womb, he is nurtured by an organ called the placenta. The placenta has the job of ensuring that the developing baby receives the nutrients and oxygen he needs to grow and survive, and also filters waste away from the baby. This organ produces a hormone referred to as inhibin A as a normal part of pregnancy, and doctors can run tests to determine how high these levels are in a woman's blood. When the levels are abnormally high, this can mean the baby has an abnormality or birth defect. Further tests are usually necessary, however, to determine whether something is actually wrong or if the levels are innocently high.

Since inhibin A testing cannot determine whether or not a baby has an abnormality, it is used as part of a screening test called the quad screen. This test checks for not only this hormone, but also for a protein called alpha-feta protein, estrogen, and human chorionic gonadotropin (hCG), which is also known as the pregnancy hormone. Doctors cannot diagnose an abnormality based on high levels of any of these substances, so the quad screen isn't considered a diagnostic test. Instead, doctors use the levels they detect in an expectant's mother's blood to decide whether a mother needs further diagnostic testing.

Interestingly, pregnancy screenings for abnormalities such as Down syndrome are thought to be more accurate when inhibin A testing is included. Another type of screening, called the triple screen, excludes testing for this hormone. It can be a useful test, but it is less likely to reveal the need for further testing when it is actually warranted. Additionally, tests that include screening for this hormone generally have a low rate of false positives.

The ovaries also secrete inhibin A as a normal part of a woman's menstrual cycle. The hormone is released by a follicle, which is the fluid-filled sack in which an egg matures, and the corpus luteum, which is the leftover follicle after a woman ovulates. Sometimes elevated levels of ovary-produced inhibin A is an indication of a tumor, especially in women who have reached menopause.