Pregnancy is a process that is controlled by a complex interaction between female sex hormones. The tiny embryo produces a female sex hormone called human chorionic gonadotropin. A standard pregnancy test detects this hormone in the urine or bloodstream.

Human chorionic gonadotropin signals the ovaries to stop ovulating and prevents the uterus from shedding the endometrium. This is why lack of menstruation is one sign of pregnancy. It also stimulates the placenta to produce another hormone, called progesterone. This hormone prevents the uterus from contracting, which would disturb the pregnancy. These hormones are also necessary in order to protect the uterus so it can provide nutrition to the developing fetus throughout the term of the pregnancy.