Five years ago, singer Kumi Koda caused an uproar when she joked on a late-night radio show about how a woman's amniotic fluid (yōsui) becomes "spoiled" as she gets older. The subtext of the comment was the advantage of having babies at a younger age, but those quick to ridicule Koda's lack of gynecological expertise (including this column) fixated on what they saw as a swipe at women who put off having children due to careers and other exigencies of life.

If Koda made the remark today, she might not receive such a negative response. Though amniotic fluid does not "spoil" as one gets older simply because it does not remain in a woman's body over time, the notion that it is more difficult for older women to have children is now an intensely discussed health issue in Japan, where the birthrate continues to decline.

And while Koda's grasp of biology was flawed, her premise — that an essential feature of a woman's ability to procreate becomes obsolete with age — wasn't far from the mark. Ovarian follicles, which produce eggs for reproduction, are present in a female's body before birth, and decrease in number as she gets older. Scientists estimate that the average girl has about 300,000 follicles at the onset of puberty, and thereafter loses about 1,000 with each menstrual cycle. When the number reaches 25,000, the chances of becoming pregnant drop considerably, which happens at about age 37, depending on the individual.