Estradiol (E2)

Estradiol (E2) is secreted by granulosa cells of growing ovarian follicles. During a natural cycle, rising estradiol signals the pituitary to trigger an LH surge and ovulation. During IVF stimulation, serial E2 measurements (alongside ultrasound) guide gonadotropin dose adjustments.

A baseline E2 should be low (under ~80 pg/mL) on cycle day 2–3; elevated baseline E2 may indicate a residual cyst or poor response. As follicles grow, E2 rises roughly 100–200 pg/mL per mature follicle. Very high E2 (over 3,000–5,000 pg/mL) increases the risk of ovarian hyperstimulation syndrome (OHSS). In FET cycles, estradiol is given as medication to build the endometrial lining.