In the first trimester, levels of human chorionic gonadotropin, a pregnancy hormone commonly known as hCG, are supposed to increase over time in normal pregnancies. Usually, hCG levels are expected to double every two to three days in early pregnancy.
hCG is made by placental cells that provide nourishment to the egg after it has been fertilized and attaches to the uterine wall. It is the same hormone that home pregnancy tests can detect in urine about 12 to 14 days after conception.
Blood tests are more sensitive and can detect the hormone as early as 11 days after conception.
In most healthy pregnancies, hCG levels, measured in milli-international units per milliliter (mIU/ml), will double about every 72 hours, which is why physicians will order two consecutive tests taken two to three days apart. hCG will reach its peak level in the first 8 to 11 weeks of a pregnancy, after which it will decline and level off for the rest of the pregnancy. Further into a pregnancy, when hCG levels are higher, it can take up to 96 hours for them to double.
After a pregnancy loss has occurred, hCG levels will return to a nonpregnant range (less than 5 mIU/ml) between four and six weeks later. However, the time it takes for hCG levels to return to this range may depend on how the loss occurred, for example, whether by spontaneous miscarriage or dilation & curettage (D&C) as well as how high the levels were when the miscarriage occurred.
It is typical for physicians to continue to test hCG levels after miscarriage until they return to the nonpregnant range.