Girls who begin menstruation earlier than average have a greater chance of being short and fat and are at greater risk for breast cancer and endometrial cancer later in life.

Now scientists have found two genes involved in determining the onset of menstruation. The study also found genes related to the onset of menopause. Early menopause increases risk of osteoporosis and cardiovascular disease.

Further research into these genes could improve understanding of all the related diseases, researchers said. The findings were reported in the journal Nature Genetics today.

Some researchers think puberty is starting earlier these days, but the idea is controversial. Puberty has been known, rarely, to start before 8 in girls or age 9 in boys.

The study, involving several institutions in the United States and Europe, analyzed data on 17,510 women across eight different international population-based sources. This number included women of European descent who reported the age at which they reached menstruation of between 9 and 17 years.

One in 20 females carry two copies of each of a gene variations that result in menstruation starting earlier, and they will start menstruating about 4.5 months earlier on average than those with no copies of the gene variants, the study found.

"This study provides the first evidence that common genetic variants influence the time at which women reach sexual maturation," said Anna Murray from the Peninsula Medical School. "Our findings also indicate a genetic basis for the associations between early menstruation and both height and BMI."

Body Mass Index (BMI) is a measure of weight as compared to height.

"The study takes us nearer to understanding the biology of the processes involved in puberty and early growth and to understand what constitutes 'normal' in growth and development," Murray said.

"Understanding the biological mechanisms behind reproductive lifespan may also help inform us about associated diseases that affect a lot of women as they get older, including diabetes, heart disease and breast cancer."

Researchers from Harvard School of Public Health (HSPH), Brigham and Women's Hospital (BWH), National Cancer Institute (NCI), and the Broad Institute of Harvard and MIT were involved in the work.