Why a Fetus Has More Eggs Than a Woman in Her 20s

Sex-educated adults are generally aware that female egg counts drop with age, but not all understand just how high their egg count is at its peak or when that peak occurs. A female's egg count peaks while she is still a fetus, at the beginning of the second trimester of pregnancy.

"Fetal females in utero by 20 weeks of gestation have close to 6 [million] to 7 million eggs. By the time of birth, that number has significantly decreased to about 2 million. The number of eggs decreases and are absorbed as the female ages. By puberty, they may have 300,000 to 400,000 eggs," explained Kecia Gaither, M.D., board-certified in OB-GYN and maternal-fetal medicine, and director of perinatal services at NYC Health + Hospitals/Lincoln in the Bronx in New York City.

"Of all the cells in the body, the only ones that are overproduced and discarded are the brain and reproductive cells. The others increase in number as we grow, such as the number of cells that make up the tissues of muscles, bone, skin, etcetera," said Sheldon Zablow, M.D., a child psychiatrist and assistant professor at the University of California San Diego Medical School.

While other cells in the body reproduce at basically an infinite rate that slows toward the end of our lives, our brain and reproductive cells are in finite supply, Zablow explained. It makes sense that neural and sexual cells are unique, because they're the basic essentials for getting through life long enough to make a new one.

But where do all those eggs actually go in the years before we reach puberty?

Prepubescent girls lose 10,000 eggs a month

"More than 10,000 eggs die each month before puberty," said Christina Burns, a licensed doctor of Chinese medicine with board certification in Oriental reproductive medicine, and founder of Naturna, an integrative health center dedicated to the comprehensive fusion of Eastern medicine and Western science in New York City.

"When a girl isn't menstruating, the eggs just die off. Essentially, they dissolve and are reabsorbed by the body. The process of dissolution is called atresia, and there hasn't been any research to tell us what happens with the dissolved cells other than they are reabsorbed by the body," Burns said.

Atresia is a medical term used to encompass several conditions. It can be the result of a malformation, such as intestinal atresia, when an obstruction prevents digestion, or aural atresia, where the ear canal is underdeveloped. Ovary follicle atresia, however, is a normal and necessary process of the ovaries to regulate the number of follicles.

This exorbitant population of immature oocytes, or unfertilized eggs, resides in the fetal ovaries and declines most rapidly before birth. This begs the question, what do we know about fetal ovaries?

The mysteries of fetal ovarian development

Though only a small number of studies have been conducted on fetal ovaries due to the complexities involved in prenatal research, the findings are interesting.

Ovaries are not uniform in shape, according to a 2011 study published in the International Journal of Applied & Basic Medical Research. Thirty ovaries from prenatal embryos and fetuses ages 6 weeks to 40 weeks and 50 postnatal ovaries up to 55 years of age were examined.

The study indicated that oval shape was predominant in prenatal ovaries (66.68 percent), followed by rod (20 percent), almond and "S" shapes (6.66 percent each). Among the postnatal ovaries, almond shape accounted for 72 percent and oval shape for 28 percent.

The researchers concluded that variations in morphological (form or structure) and morphometric (shape and dimensions) development characteristics of ovaries are common.

When a girl isn't menstruating, the eggs just die off. Essentially, they dissolve and are reabsorbed by the body.

Placement is another element that differs between fetal and adult ovaries. During the first trimester of development, "the fetal ovaries are located on the posterior abdominal wall and descend to [the] pelvis as development progresses," stated the 2011 study authors.

This ovarian migration has raised additional questions. For example, scientists debate whether the right and left fetal ovary descend at the same rate or not. A 2006 study suggested differences in descent, while the 2011 study did not.

Much is still unknown in the realm of fetal ovarian and follicular development, and it remains a difficult area to study. But it's incredible to realize that when a female baby is delivered, the egg that may one day be fertilized and become the new mother's grandchild is already present in the ovaries of the newborn.

The developing knowledge in this area may have practical future use in identifying potential ovarian conditions before they develop. For the rest of us, it's just fun to learn.