Expansion
Oocyte number is fixed before birth and declines continuously thereafter
Expansion
Numbers
- About 7 million oogonia at 20 weeks of gestation
- About 1 to 2 million at birth
- About 300,000 at puberty
- About 1000 at menopause
- Only about 400 are ever ovulated
Unlike spermatogenesis, no new oocytes are formed after birth.
Meiotic arrest: oocytes arrest in prophase I before birth and complete the first division only at ovulation, decades later. The second division completes only at fertilisation. This prolonged arrest is the accepted explanation for the steep rise in non-disjunction and therefore aneuploidy, notably trisomy 21, with maternal age.
Markers of ovarian reserve
- Anti-Mullerian hormone: produced by small antral follicles, stable through the cycle and the best single marker
- Antral follicle count on ultrasound
- Early follicular FSH: rises late, so it is a less sensitive marker
Fertility declines gradually from the early thirties and steeply after 37, and the fall in oocyte quality matters more than the fall in number.