corpus luteum of pregnancy

The corpus luteum of pregnancy is the structure that keeps a brand-new pregnancy alive before the placenta is ready to take charge. Think of it as a temporary life-support generator, the same one left over after ovulation, but kept running past its usual shutdown date by a signal from the embryo.

After an egg is released, the leftover follicle becomes the corpus luteum, which secretes progesterone. Normally, if no pregnancy occurs, it withers after about two weeks and progesterone falls, triggering menstruation. But if an embryo implants, its hCG signal rescues the corpus luteum, so it persists and continues producing the progesterone (and some estrogen and relaxin) needed to keep the uterine lining intact.

This rescue is only a stopgap. Around weeks seven to ten of pregnancy, the placenta becomes capable of producing enough progesterone on its own, and the corpus luteum gradually fades. This handover is called the luteal-placental shift.

A clinical caveat: because the early pregnancy depends on corpus luteum progesterone, surgical removal of the corpus luteum before the placental shift can threaten the pregnancy, which is why timing matters in early-pregnancy surgery and in interpreting progesterone support.