At what age does fertility begin to decrease?
Girls are born with a fixed number of immature eggs in their ovaries. The number of eggs decreases as women get older.
At birth, most girls have about 2 million eggs, at adolescence that number has gone down to about 400, 000, at age 37 there remain about 25,000.
By age 51 when women have their menopause they have about 1000 immature eggs but these are not fertile.
At every menstrual cycle one of the immature eggs will mature and be released during ovulation. The eggs that are not released die and get re-absorbed into the body.
The quality of the eggs also gets poorer as women get older. All other things being equal the number and quality of the woman’s eggs determines her fertility.
Adapted by permission from BMJ Publishing Group Limited. [Delaying childbearing: e ect of age on fecundity and outcome of pregnancy, van Noord-Zaadstra et al., 302, p. 1363, 1991]