Corpus luteum regression and menstruation
Without pregnancy, the corpus luteum (the 'yellow body' left after ovulation) shrinks into a scar, so progesterone falls and menstruation, the shedding of the womb lining, begins.
Remember
Without β-hCG (a hormone made by an implanted embryo), the corpus luteum is not rescued and becomes the corpus albicans, a scar that makes no progesterone, so the womb lining is shed. Progesterone falls first and menstruation follows.
Memory aid
yellow means working and white means finished
The yellow body makes progesterone and the white body does not.
Once the egg has been released at ovulation, the sac that held it (the follicle) collapses and its cells form the corpus luteum, literally the 'yellow body'. It is a temporary, hormone-producing gland, and it makes progesterone along with some oestrogen. The endometrium, which is the lining of the womb, is held thick by progesterone, ready for an embryo to implant.
In a pregnancy, the implanted embryo makes β-hCG, which rescues the corpus luteum. Progesterone then stays high and there is no period. With no pregnancy, no β-hCG arrives and, after roughly 10 to 14 days, the corpus luteum regresses. That means it shrinks and dies back.
What it leaves behind is the corpus albicans, literally the 'white body': essentially scar tissue that does not release progesterone. Progesterone and oestrogen fall as the corpus luteum regresses, and the endometrium loses its support, breaks down and is shed. That is how the lining comes to be shed as menstruation.
The order matters. Progesterone falls first, and the bleeding follows.
Key facts
- Yellow body
- the corpus luteum is a temporary, hormone-producing gland that makes progesterone along with some oestrogen after ovulation.
- White body
- the corpus albicans is the scar left when the corpus luteum regresses, and it does not release progesterone.
- Pregnancy rescue
- β-hCG from the implanted embryo keeps the corpus luteum, so progesterone stays high and there is no period.
- Timing
- with no β-hCG, the corpus luteum regresses roughly 10 to 14 days after ovulation.
- Order of events
- the corpus luteum regresses and progesterone falls (the corpus albicans is the scar left behind), then menstruation follows.
Exam traps
- A corpus luteum kept going by β-hCG is what happens in early pregnancy, not when there is no pregnancy, because β-hCG is made by an implanted embryo and, with no pregnancy, it is not produced.
- The corpus albicans does not keep oestrogen production going, because it is nonfunctional scar tissue that makes no oestrogen. As the corpus luteum regresses, oestrogen and progesterone both fall, and the lining thins and is shed.
- Menstruation does not come first and then make the corpus luteum regress: the lining is shed because progesterone support has been withdrawn, so the hormone change comes first.
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