Female fertility is a remarkable biological journey that begins long before a girl is born and continues through puberty, adulthood, and eventually ends at menopause. This fertility timeline is shaped by hormonal cycles and natural declines in egg count, making it a key aspect of women’s health that evolves over decades.
During early foetal development, a baby girl already begins forming her lifetime supply of eggs (known as oocytes). By around 20 weeks of gestation, a female foetus has approximately 6 million eggs in her ovaries. However, this number declines rapidly—even before birth. By the time she is born, only 1 to 2 million eggs remain. This dramatic reduction continues through childhood, with more than 10,000 eggs dying each month before puberty begins.
At puberty, the average girl has about 300,000 to 400,000 eggs left. This is when the menstrual cycle starts, usually between the ages of 8 and 15, with an average onset around age 12. The brain’s hypothalamus begins producing gonadotropin-releasing hormone (GnRH), which signals the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones trigger the ovaries to begin maturing eggs and producing oestrogen, resulting in menstruation.
Each menstrual cycle, between 10 to 20 eggs (sometimes up to 30) are selected from the pool of immature eggs (oocytes) in the ovaries. These eggs start to mature in fluid-filled sacs called follicles activated by FSH. However, only one egg (occasionally two, in the case of fraternal twins) becomes the dominant follicle that fully matures and is released during ovulation. The rest of the eggs in that group undergo a natural degeneration process known as atresia. Regardless of ovulation or birth control use, women continue to lose about 1,000 eggs per month.
By age 37, most women have around 25,000 eggs remaining. This is around the tie that some women may already start to experience perimenopausal symptoms. Perimenopause, the transitional phase before menopause, is marked by fluctuating hormones and unpredictable periods. At menopause, which typically occurs around age 51, fewer than 100 viable eggs remain. Hormone levels of estrogen and progesterone decline sharply, causing symptoms like hot flushes, mood swings, and irregular cycles.
Once ovulation ceases completely, menopause signals the end of a woman’s natural fertility.
Understanding this timeline empowers women to make informed decisions about reproductive health, fertility preservation, and hormonal wellness throughout every life stage.
