Although I was born in 1976, the egg that I came from was created in my grandmother’s womb– somewhere in Georgia, in 1946.

Want to calculate the vintage of the egg you came from? Take your mother’s date of birth and subtract about 20 weeks.

That’s true because unlike males, who constantly generate sperm after they hit puberty, girls are born with their one and only lifetime supply of eggs. Around the 20th week of gestation, a female fetus has developed a reproductive system, including 6 to 7 million eggs in her ovaries.

The matrilineal line looks much like a nested Russian doll.

The egg that created you was formed inside of your mother’s fetus while she was inside of your grandmother’s womb.

UPDATE: A few comments have pointed to a recent study in mice that suggests the possibility that a woman’s supply of eggs might be replenished. Questions remain about this controversial finding that scientists are working to understand, verify and replicate. We think it’s premature to revise the textbooks, but it’s exciting to consider new possibilities in science and to follow how they are validated or refuted by the scientific process. This finding, if true, could have implications for fertility treatment, so we’ll stay tuned!