The follicular phase is the first half of the ovarian cycle. It begins on day 1 of menstruation and ends with ovulation, and it is the most variable portion of the menstrual cycle — typically 12–21 days — which is why total cycle length differs from person to person. While the period is occurring, the brain is already preparing the next egg: in the final days of the preceding luteal phase, falling progesterone and inhibin allow follicle-stimulating hormone (FSH) to rise, recruiting a cohort of antral follicles to grow. During this phase, FSH stimulates a group of small follicles, and through a process of competitive selection a single dominant follicle — the one with the greatest FSH sensitivity — emerges by about day 5–7. This dominant follicle becomes the engine of the phase: it secretes rising amounts of estradiol, which thickens the uterine lining (the proliferative endometrium), thins cervical mucus to favor sperm, and feeds back on the pituitary. At low and moderate levels estrogen suppresses FSH and LH (negative feedback), pruning the rest of the cohort to atresia; but...