Random follicle selection limits twins

A mathematical model suggests the first ovarian follicle is chosen at random once follicle‑stimulating hormone (FSH) crosses a threshold; estradiol produced by that follicle quickly suppresses FSH and shuts the window for selecting others, so most cycles yield one ovulation and fraternal twins remain rare. The model quantifies the timing window and predicts higher twin probability when the feedback control loosens (e.g., older maternal age) or lower selection when FSH never reaches threshold (e.g., some polycystic ovary syndrome cases). — This reframes parts of fertility science and clinical messaging by replacing a 'biggest follicle wins' story with a stochastic, timing‑based mechanism that changes how we think about twin risk, age effects, and infertility causes.