Young ballet dancers and other skinny teenage girls who exercise strenuously tend to go through puberty a year or more later than normal, a biologist has found. In a study conducted at three Ballet schools in New York, Dr. Rose E. Frisch found that some of these hard-driven girls had not had their first menstrual cycle by the time they reached their late teens. "Ballet dancers worship thinness," she said, and this, combined with their heavy exercise, apparently disrupts normal sexual maturation. "There is quite a bit of evidence that athletes display the same kind" of effect, she said. "What we don't know yet is why this is happening. And we don't know if it is harmful in any way."

The survey covered 89 girls whose average age was 16. It found that 20 had not had their first period, including one who was nearly 21 years old. Of those who had had a period, the first cycle occurred at an average age of 13.7, about a year later than usual. Only a third of the girls had regular periods, while the rest reported irregularity or prolonged stretches without. The girls averaged 5 feet 4 and 100 pounds. Most had been training since age 6 or 7. Frisch theorized that late or disrupted menstruation is nature's way of preventing pregnancy during times of famine. "There's no question that pregnancy takes about 50,000 calories over and above normal demands and lactation takes 1,000 calories a day," she said. "Normally, the woman stores fat because in prehistoric times, if she didn't store the energy in advance, when there were fluctuations in the food supply, she would abort."