In which situation would hCG levels be lower than expected for the gestational age?

Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy, and its levels can vary depending on the situation. In the case of an ectopic pregnancy, the levels of hCG are often lower than what would be expected based on the gestational age. This occurs because the developing embryo is not correctly implanted in the uterus, which can lead to inadequate production of hCG.

In a normal intrauterine pregnancy, hCG levels typically rise rapidly during the early weeks of gestation as the placenta develops and takes over hormone production. However, with an ectopic pregnancy, the hormone may not rise as expected since the placental tissue is not developing properly or is less functional outside the uterus.

In contrast, other conditions such as choriocarcinoma and hydatidiform moles, which are forms of gestational trophoblastic disease, usually result in higher than expected levels of hCG due to abnormal proliferation of trophoblastic cells. Twin gestation often results in elevated hCG levels as well, reflecting the presence of multiple placenta and fetal units, which produce more of the hormone. This is why ectopic pregnancy is recognized as the condition associated with lower hCG levels relative to gestational age.