Which hormone's presence indicates that pregnancy is occurring?

Chorionic gonadotropin, also known as human chorionic gonadotropin (hCG), is the hormone produced by the placenta shortly after implantation occurs in pregnancy. Its presence in the blood or urine is a primary indicator of pregnancy, which is why pregnancy tests are designed to detect hCG levels.

After implantation, hCG serves crucial functions, such as maintaining the corpus luteum, which in turn continues to produce progesterone to sustain the uterine lining and prevent menstruation. This is essential in the early stages of pregnancy until the placenta can take over hormone production. The rise in hCG levels is typically what is measured in pregnancy tests and is a vital part of diagnosing pregnancy.

In contrast, while estrogen and progesterone are important for maintaining the pregnancy, their presence is not solely an indicator of pregnancy itself, as they can also be present in other contexts, such as the menstrual cycle. FSH (follicle-stimulating hormone) plays a role in regulating the reproductive processes, but it is not directly involved in indicating pregnancy. Therefore, hCG is the most definitive marker for establishing whether a pregnancy is occurring.