At birth, a female baby is born with all the eggs she will ever have in her lifetime. These eggs are contained in structures called follicles within the ovaries. The number of follicles and eggs present at birth determines a woman’s reproductive potential later in life.
Total Number of Eggs at Birth
Research has shown that baby girls are born with approximately 1-2 million eggs in their ovaries. This is the total number of eggs that is peaked at 20 weeks gestation in the developing fetus. After 20 weeks, the number of eggs begins to decline rapidly. By birth, the number has dropped to around 1-2 million.
This high number of eggs present early in development represents the total ovarian reserve available to a female throughout her reproductive life. Out of the millions of eggs, only around 300-400 will ever mature and be ovulated during the woman’s reproductive years. The rest of the immature eggs remain quiescent in the ovarian follicles, before undergoing atresia (degeneration) prior to puberty or throughout the female’s life.
Factors Affecting Total Egg Number
While the average number of eggs present at birth is 1-2 million, there can be variation among different females. Some of the factors that can influence the total number include:
- Genetics – Studies in mice have identified certain genes that may be involved in determining egg number. Women may inherit differences genetically in their initial egg endowment.
- Ethnicity – Some research has observed differences in ovarian reserve between ethnicities. For example, African-American and Hispanic women may have lower egg reserves compared to Caucasian women.
- Birth weight – Higher birth weight in babies has been associated with a larger pool of primordial follicles in the ovaries.
- Gestational age – Full term babies (37-42 weeks) tend to have more follicles than premature babies.
- Environmental factors – Exposure to environmental toxins like cigarette smoke while in the mother’s womb may reduce egg reserves.
Decline of Egg Supply Over Time
The 1-2 million eggs present at birth represent the complete supply available to sustain a woman’s fertility. This egg reserve declines progressively from puberty onwards:
- At puberty, there are 300,000 to 500,000 eggs remaining
- By age 30, there are around 150,000 eggs left
- By age 40, only around 25,000 eggs remain
- At menopause, the remaining eggs are depleted
The decline accelerates as a woman reaches her mid-30s, as more eggs undergo atresia. Women are born with all the oocytes they will ever have, and the decline in ovarian reserve with age is irreversible.
Rate of Decline
Research has tried to estimate the rate of decline in egg reserves with age:
- From birth through puberty, there is an estimated loss of around 100,000 eggs per month.
- During the reproductive years, the rate of decline is an estimated 1,000 – 2,000 eggs lost per month.
- In the 5-10 years before menopause, this rises to over 10,000 eggs lost per month.
The accelerated phase of egg loss in the years prior to menopause is thought to underlie the age-related decline in female fertility.
Assessing Ovarian Reserve
Since egg reserves cannot be directly counted, ovarian reserve testing is used to estimate the remaining supply. Common tests include:
- Antral follicle count – ultrasound to count small antral follicles
- AMH level – anti-mullerian hormone level
- FSH level – follicle stimulating hormone level
These tests can indicate if a woman has low ovarian reserve compared to others of the same age. Low reserve is associated with reduced fertility potential and earlier menopause.
Why are Millions of Eggs Present at Birth?
The presence of a huge surplus of eggs from the start of life, far more than will ever fully mature, suggests that there is an evolutionary advantage to this strategy. Some of the proposed benefits include:
- Maximizing reproductive lifespan – Even with substantial egg loss over time, enough eggs remain for fertility into the 40s.
- Providing insurance against defects – The large initial pool allows defective eggs to be weeded out while leaving some functional eggs.
- Response to high infant/child mortality – Many eggs allow women to replace lost children by conceiving again.
- Risk spreading – Having many eggs reduces risk if some are lost prematurely.
The high initial egg endowment buffer women’s fertility against the significant oocyte loss that will occur throughout life. It supports human reproductive success across the decades-long lifespan.
Implications of Egg Supply at Birth
The finite pool of oocytes endowed at birth has implications for a woman’s reproductive health and options:
- Once eggs are lost, they cannot be regenerated. The initial number defines the limits of fertility.
- Loss of eggs over time contributes to decreased fertility, risks of chromosomal abnormalities, and eventual menopause.
- Younger age is associated with higher remaining reserves and fertility potential.
- Older women have drastically reduced egg supplies, making conception less likely.
- Egg freezing at a younger age preserves fertility potential.
- Donor eggs may enable pregnancy when a woman’s own supply is depleted.
Understanding the initial oocyte endowment and pattern of decline can help women make informed reproductive choices.
Conclusion
In summary, baby girls are born with a large stockpile of around 1-2 million eggs contained in primordial follicles in their ovaries. This represents the complete lifetime supply, as no new eggs are made after birth. Only a tiny fraction of these eggs will ever be used, with the rest progressively lost. The initial high number buffers against defects and facilitates fertility into the 40s, despite enormous attrition. Ovarian reserve testing can estimate remaining eggs. Knowledge of the finite egg supply from birth and its age-related decline can help women understand the biology behind their reproductive options and risks.