When fertilisation occurs and fetus is implanted
in the endometrium, the trophoblast cells of the developing
placenta secrete a hormone called human chorionic
gonadotrophin (hCG). This hormone like LH maintains the
corpus luteum and secretion of progesterone. The absence of
menstrual bleeding is the first sign of pregnancy. By the 16th
week of pregnancy, the placenta produces enough progesterone
for a normal pregnancy.