Women who smoke heavily during pregnancy tend to have daughters who start menstruating months earlier than the daughters of women who didn't smoke while pregnant, a new study finds.
The trend was equally true in mothers who smoked for part of their pregnancy and then quit.
However, girls whose mothers smoked at least 10 cigarettes per day during pregnancy, or smoked for part of their pregnancies, began menstruating only 3 to 4 months earlier than girls whose mothers didn't smoke at all, which is not a huge difference, said Dr Gayle Windham of the California Department of Public Health.
"For any one kid, a few months isn't going to make much of a difference, probably," she said. However, if large groups of girls began menstruating earlier, even by just a few months, that could have an overall effect on the population, she added.
Research has shown that women who start menstruating early may be at higher risk of health problems later in life.
More likely to die
A large study of Norwegian women, for instance, found that those who began menstruating before age 12 were slightly more likely to die during a 37-year study period than girls who didn't start their peers who'd begun menstruating at age 14.
Early menstruation has been tied to a number of health risks, including heart disease, breast cancer and even lung problems such as asthma.
Earlier periods have also been associated with earlier sexual activity and use of substances such as cigarettes and alcohol, Dr Anshu Shrestha of the University of California, Los Angeles, said.
During the study, Shrestha and colleagues reviewed data collected from 13,815 pregnant women in two Danish cities between 1984 and 1987 about their use of alcohol and cigarettes.
In 2005, the authors contacted all the female children of these mothers to ask them about the timing of their first period. More than 3,000 women responded, and about half remembered the exact month and year they began menstruating. On average, the girls in the study got their first period just after they turned 13.
Honesty in report a great concern
One concern, Shrestha noted, is that the study relied on women to report how much they smoked and drank during pregnancy, and people often underreport any behaviour they believe will be frowned upon.
However, the data about pregnancy were collected in the 1980s, when it was considered "part of the norm" to smoke and drink, she said. "Therefore, we have no reason to believe (there was) any underreporting."
Indeed, more than 40% of mothers said they smoked while pregnant, and 70% said they drank at least 1 alcoholic beverage per week, with 17% binge drinking at least once during pregnancy.
The authors found no association between early menstruation and exposure to smoke during childhood, maternal smoking before pregnancy and maternal alcohol drinking during pregnancy, they reported in Human Reproduction.
Shrestha said that she wasn't surprised to see no link between drinking in pregnancy and the age daughters began menstruating, since very few mothers in the study said they drank heavily while pregnant.
It's not yet clear why exactly smoking harms the foetus, but smoking by-products cross the placenta, and smoking exposure has been linked to changes in hormone levels in the babies once born, which may be triggering the early periods in girls, Shrestha noted.
Windham agreed: "I would think it's most likely a hormonal effect."
And even though most pregnant women no longer smoke, some continue to do so, Dr Shrestha said. Recent US data showed that 1 out of every 10 women smoked during pregnancy and similar rates have been reported in other developed countries.
(Reuters Health, Alison McCook, December 2010)