Oocyte Counts Across Female Life Stages

 

Fetal Development: In humans, all oocytes (eggs) are formed prenatally – no new eggs are made after birth. During fetal life germ cells proliferate and differentiate into primordial follicles. Classical teaching is that a female fetus reaches a peak of ~6–7 million oocytes total (roughly 3–3.5 million per ovary) by mid-gestation. However, histological modeling suggests the average peak is much lower (~300,000 primordial follicles per ovary at ~18–22 weeks). After this peak enormous atresia ensues: by birth the count has fallen by over half.

Figure: Early ovarian follicles in a fetal ovary (primordial follicles with oocyte in center), illustrating the huge pool present before birth.

At Birth: Just before birth, each ovary still holds on the order of hundreds of thousands of eggs. A commonly cited range is ~1–2 million oocytes total at birth (roughly 0.5–1.0 million per ovary). More recent quantitative models estimate ~295,000 nongrowing follicles per ovary at birth(≈590,000 total). Thus a newborn girl typically has on the order of half a million eggs per ovary. From birth onward, the oocyte pool monotonically declines (no new eggs are created. By puberty only ~10–20% of the birth‐time pool remains, due to continuous atresia.

Puberty and Adolescence: By the time of puberty (menarche), the ovarian reserve has dropped dramatically. Standard references cite about 300,000–500,000 total oocytes at puberty (on the order of 150,000–250,000 per ovary). This implies each ovary holds only a few hundred thousand eggs at puberty. The rate of loss slows after puberty sets in, in part due to hormonal regulation of follicle recruitment. In adolescence the ovarian cortex contains mostly quiescent primordial follicles; every month, a cohort of follicles (hundreds to around a thousand) is recruited to grow, but only one dominant follicle will usually ovulate, while the rest degenerate (atresia).

Reproductive Years (20s–30s): During a woman’s reproductive years the ovarian reserve continues to decline gradually. By age 30 only a small fraction (≈10–15%) of the original pool remains. For example, one model predicts that by age 30 women retain only ~12% of their peak prenatal reserve. Concretely, there may be on the order of 100,000 total follicles left by age 30 (perhaps ~50,000 per ovary), though numbers vary widely. By the mid-30s the count is even lower; by age 37 roughly 25,000 total oocytes remain(≈12,500 per ovary). A small fraction of these are in the resting pool; most follicles are small pre-antral or early antral stages. Each menstrual cycle still recruits on the order of ~1,000 follicles, but only one (occasionally two) will reach ovulation. In each cycle nearly all non-dominant follicles undergo atresia. Over a woman’s lifetime only on the order of 300–500 oocytes will ever fully mature and ovulate – the vast majority (>99%) are lost to follicular atresia rather than ovulation.

Figure: A maturing ovarian follicle (primary/secondary follicle) during reproductive years. Each menstrual cycle many follicles begin to grow but typically only one reaches ovulation.

Perimenopause and Menopause: As women enter their 40s, the oocyte pool nears exhaustion. Menopause (cessation of menses) occurs when only a few hundred to a thousand total follicles remain. On average, about age 50–51 is menopause, at which point roughly 1,000 oocytes in total (≈500 per ovary) are left. In other words, by menopause the ovarian reserve is nearly depleted – only a few primordial follicles survive, and basically no functional eggs remain. (For perspective, ~450 ovulatory cycles occur in a typical reproductive span; each cycle uses one egg, so only a few hundred of the initial pool are ever ovulated.

Follicle Loss and Ovulation: From conception onward the oocyte pool continually shrinks by atresia. Prenatally, more than half of fetal oocytes die before birth. Postnatally, a steady attrition continues: every menstrual cycle recruits dozens–hundreds of follicles to grow, but only one reaches full maturity. Most of the recruited cohort (e.g. up to ~1,000 initial follicles in some cycles) undergoes atresia. Over decades, this process leaves only a few hundred eggs to ovulate; as noted, only ~300–500 eggs mature during a woman’s lifetime. Thus atresia is relentless – at each age the majority of the pool is lost to follicle death rather than ovulation. By early 30s only a few percent of the prenatal maximum remain, and by 40 virtually all are gone.

Ovary Laterality and Biological Variations: In general, the two ovaries carry very similar egg reserves. However, subtle asymmetries exist. For example, one large clinical study found the right ovary tends to ovulate slightly more often than the left (about 55% of ovulations were right-sided vs 45% left). Consistent with this, ultrasound surveys show the right ovary is on average slightly larger and has about 8–10% more antral follicles than the left. These differences are modest and of uncertain significance for fertility. Importantly, there is tremendous individual variation in ovarian reserve. Models incorporating histological data suggest that the number of follicles at birth (per ovary) can range widely: e.g. ~35,000 in women destined for early menopause up to >2.5 million in those with late menopause. Women inherit different peak counts and lose follicles at different rates (influenced by genetics, environment, etc.), so the “typical” numbers above have wide confidence intervals. Nonetheless, the overall pattern holds: a fetal peak in the millions (total), a decline to hundreds of thousands by birth and puberty, and eventual depletion to nearly zero by menopause.

Sources: Authoritative medical and scientific reviews provide these estimates (e.g. ACOG guidance and peer-reviewed ovarian‐reserve studies.

The numbers above are averages – individuals vary widely in their ovarian reserve.

 

References:

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8. Larsen, E. C., et al. (2013). Ovarian reserve assessment and impact of fertility preservation: A review. Human Reproduction Update, 19(5), 514–527. https://doi.org/10.1093/humupd/dmt020

9. te Velde, E. R., & Pearson, P. L. (2002). The variability of female reproductive ageing. Human Reproduction Update, 8(2), 141–154. https://doi.org/10.1093/humupd/8.2.141

10. American College of Obstetricians and Gynecologists (ACOG). (2014). Ovarian reserve testing. Committee Opinion No. 618. https://www.acog.org