Women who smoke heavily during pregnancy tend to have daughters who start menstruating months earlier than the daughters of women who did not smoke during their pregnancy.

Previous studies have found evidence that women who start menstruating early may be at higher risk of health problems later in life. 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.

To assess whether exposure to prenatal smoking or alcohol accelerates age of menstrual cycle in the baby, the researchers 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 researchers contacted all the female children of these mothers to ask them about the timing of their first menstrual period. A total of 3,169 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 years. More than 40 percent of mothers said they smoked while pregnant, and 70 percent said they drank at least 1 alcoholic beverage per week, with 17 percent binge drinking at least once during pregnancy.

No association was found between early menstruation and exposure to smoking during childhood, mother's smoking before pregnancy as well as mother's alcohol consumption during pregnancy. However, the women who smoked heavily during pregnancy had daughters who got their menstrual cycles at least 3 months before those whose mothers did not smoke during pregnancy.

It's not yet clear why exactly smoking harms the fetus, but smoking byproducts cross the placenta, and smoking exposure has been linked to changes in hormone levels in the babies once born, which could trigger the early periods in these girls.