Estradiol (often written E2 on lab reports) is made mainly by the follicles (fluid-filled sacs that hold eggs) 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. Each growing follicle adds to the total. So the rising level helps the doctor judge how many follicles are responding, adjust medicine doses, and time the trigger injection safely.
Outside treatment
Estradiol is also part of routine hormone blood tests for irregular periods, and it's the estrogen given as tablets or patches to prepare the uterine lining in frozen embryo transfer cycles.