The endometrium is the tissue an embryo actually implants into, which makes it as important to conception as the egg and sperm. In the first half of the cycle, rising estradiol (the main form of estrogen) makes it grow and thicken. After ovulation, progesterone makes it ready to accept an embryo. If no pregnancy occurs, it sheds — that's the period — and the cycle begins again.
In fertility care
Ultrasound checks the lining's thickness and pattern during cycle monitoring and before embryo transfer. A lining that's too thin, or changed by polyps, fibroids, or adhesions (scar tissue), 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.