Follicle

Each ovarian follicle consists of an oocyte surrounded by granulosa cells, enclosed in a fluid-filled antrum. Under FSH stimulation at the start of each menstrual cycle, a cohort of antral follicles begins growing. As they compete for dominance, one follicle (the dominant follicle) outpaces the others, reaching 18–25 mm at maturity while the rest undergo atresia.

Follicle size is monitored by transvaginal ultrasound during ovarian stimulation. In IVF, the goal is to develop multiple follicles simultaneously to 17–20 mm before triggering. The number of follicles retrieved is the main determinant of how many eggs — and ultimately how many embryos — a cycle produces. Follicle size alone does not guarantee egg maturity; only about 70–80% of retrieved follicles contain a mature MII egg.