Estradiol (often written E2 on lab reports) is produced mainly by the follicles growing in the ovary. As follicles grow through the first half of the cycle, estradiol rises — thickening the lining of the uterus (the endometrium) in preparation for a possible pregnancy.
In fertility monitoring
During IVF stimulation, estradiol is measured in blood every few days alongside ultrasound scans. Because each growing follicle contributes to the total, the rising level helps the doctor judge how many follicles are responding and adjust medicine doses — and helps time the trigger injection safely.
Outside treatment
Estradiol is also part of routine hormone panels when evaluating irregular cycles, and it's the estrogen given as tablets or patches to prepare the uterine lining in frozen embryo transfer cycles.