The total number of viable eggs available to a woman, known as her ovarian reserve, is a fixed quantity established before birth. Unlike sperm production in men, the female body does not create new eggs. The decline in both the number and quality of these eggs accelerates significantly as a woman enters her late thirties, making age 40 a defining point for understanding fertility potential and the natural biological constraints on conception.
The Biology of Ovarian Reserve Depletion
A female fetus reaches her peak egg supply at about 20 weeks of gestation, with an estimated six to eight million eggs housed within ovarian follicles. This count drops dramatically, so that by birth, only about one to two million eggs remain in the ovaries. The number continues to fall, leaving approximately 300,000 to 400,000 eggs by the time a woman reaches puberty.
The immense loss of eggs is not due to ovulation, but rather a continuous, non-replenishable process of programmed cell death called atresia. Only about 300 to 500 eggs will ever be released through ovulation during a woman’s reproductive years, while the vast majority degenerate and are reabsorbed by the body. Approximately 1,000 immature eggs are lost each month through atresia, a process that is independent of hormones, pregnancy, or birth control use.
Estimated Egg Count and Fertility Chances at Age 40
The total number of remaining eggs for a woman at age 40 is typically a small fraction of the initial reserve, often cited to be less than 3% of the pre-birth total. While individual variation is significant, the estimated range of remaining eggs is between 5,000 and 10,000. This pool represents the final stage of the quantitative decline that began decades earlier.
The diminishing quantity of eggs directly translates into a sharp reduction in the monthly probability of natural conception. In their early twenties, a healthy couple has about a 25% chance of pregnancy in any single menstrual cycle, but by age 40, this chance drops significantly. Research indicates that the probability of getting pregnant per menstrual cycle for women at age 40 is typically between 5% and 10%.
The Role of Egg Quality
At age 40, the viability of the remaining eggs becomes a greater factor in fertility than the quantity. The eggs that have been stored since before birth have aged along with the woman, which affects their cellular health. This biological aging of the oocyte leads to a much higher rate of chromosomal abnormalities, known as aneuploidy.
Aneuploidy means the egg contains an incorrect number of chromosomes, such as having too many or too few. This is primarily caused by errors in the meiotic spindle, the structure responsible for proper chromosome separation during the final stages of egg maturation. Eggs from women over 40 are significantly more likely to have this genetic error, which is the main reason for increased difficulty achieving a live birth.
These chromosomally abnormal embryos often fail to implant in the uterus or result in an early miscarriage. For women over age 40, the rate of miscarriage is markedly higher, linked directly to the elevated percentage of aneuploid eggs. The high success rate of pregnancy using donor eggs from younger women confirms that egg quality, which is intrinsically tied to age, is the primary biological barrier to conception in this age group.
Clinical Tools for Assessing Reserve
Doctors use specific clinical tests to estimate a woman’s current ovarian reserve. These tests measure the size of the remaining follicle pool and provide an estimate of the egg quantity. One common blood test measures Anti-Müllerian Hormone (AMH), a hormone secreted by the small, developing follicles within the ovary.
AMH levels tend to correlate with the number of early antral follicles present, offering a reliable, cycle-independent proxy for the overall egg reserve. Another common assessment is the Antral Follicle Count (AFC), which is a visual count of the small, resting follicles performed via transvaginal ultrasound. An AFC of five or less often indicates a diminished reserve.
A third test measures Follicle-Stimulating Hormone (FSH) levels, typically performed on day three of the menstrual cycle. High FSH levels can indicate that the ovaries are becoming less responsive and require more stimulation from the brain, suggesting a low ovarian reserve. While these tests are good predictors of the remaining quantity of eggs, they offer limited insight into the quality or genetic health of the eggs.