HCG begins to drop after a miscarriage in a biochemical pregnancy. A biochemical pregnancy is one in which the sperm-egg combination does not settle in the uterus. HCG values in biochemical pregnancies are usually not high and rise slowly. After the embryo is biochemically aborted, HCG values will begin to decline and will fall to normal shortly thereafter. There are many causes of biochemical pregnancy, including embryonic, maternal, paternal and environmental factors. Chromosomal abnormalities in the embryo are the most common cause. Most biochemical pregnancies are characterized by vaginal bleeding, which is often mistaken for menstruation because of its proximity to the onset of menstruation. Most biochemical pregnancies occur by chance, and if they are not caused by a genetic abnormality, they do not affect the next pregnancy. Patients with multiple biochemical pregnancies need to be examined to determine the cause.