March 10, 2004 -- A surprising finding about female fertility in mice is challenging the notion that women are born with a lifetime's supply of eggs.

Researchers have long believed that female mammals, including mice and humans, were born with a fixed supply of egg-producing follicles in their ovaries. In contrast, most male mammals continuously generate new sperm cells that allow them to reproduce throughout their adult life.

But a new study published in the March 11 issue of Nature shows that female mice continue to produce eggs to replace damaged ones after birth.

Researchers say the discovery indicates that females may share the ability to replenish reproductive cells during life, and if the same process occurs in humans it may also help explain why female fertility declines rapidly after age 30.