Genetic clues to age of first period
The timing of when a girl reaches puberty is controlled by hundreds of genes, say scientists.
And age at first period may vary in daughters from the same family because of genetic factors, research shows.
The findings, published in Nature, could give clues to why early puberty may be linked to an increased risk of health conditions.
Scientists at 166 institutions analysed the DNA of more than 180,000 women in one of the largest studies of its kind.
They found that hundreds of genes were involved in the timing of puberty.
Unusually, a girl's first period was also influenced by imprinted genes - a rare event where genes from either the mother of father are silenced.
"Our findings imply that in a family, one parent may more profoundly affect puberty timing in their daughters than the other parent," said lead researcher Dr John Perry of the University of Cambridge.
He said the biological complexity revealed in the study was "amazing".
"We identified more than 100 regions of the genome associated with puberty timing, but our analysis suggests there are likely to be thousands," he told BBC News.
Most girls start their periods between the ages of 10 and 15, with an average age of 12-13, according to evidence from the UK Biobank.
While genes play a role in timing, environmental factors are also important.
Factors such as childhood body mass index (BMI) and exercise have also been linked to puberty.
The researchers say the age a woman has her first period is associated with the risk of chronic diseases.
"Menarche is associated with the development of health conditions later in life in women such as diabetes, cardiovascular disease, and breast cancer," said Dr Joanne Murabito, associate professor of medicine at Boston University School of Medicine.
"By studying genetic factors we hope to better understand how puberty timing in girls is linked to important health conditions in women."