The endometrium is the tissue an embryo actually implants into, which makes it as important to conception as the egg and sperm. Through the first half of the cycle, rising estradiol makes it grow and thicken; after ovulation, progesterone matures it into a receptive state. If no pregnancy occurs, it sheds — that's the period — and the cycle begins again.
In fertility care
Lining thickness and pattern are checked by ultrasound during cycle monitoring and before embryo transfer; a lining that's too thin or structurally disturbed (by polyps, fibroids, or adhesions) can reduce the chance of implantation. In frozen embryo transfer cycles, estrogen and progesterone are given specifically to build and mature the lining before the transfer is scheduled.