More than 70 genes located on a fetus’ sex chromosomes determine whether a baby will be a male or female. Until the seventh or eighth week of development, both boys and girls have identical genitalia, called the genital ridge. A baby girl’s ovaries start to appear between weeks 11 and 12 of fetal development and produce about seven million eggs. Throughout the female’s life, the number of eggs produced continues to decline, however. At birth, the number will have already dropped to two million.

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