The age at which girls reach puberty is influenced by a set of genes that were previously only known to be involved in the development of a fetus before birth, according to a new study. Scientists are interested in understanding the factors that influence when people enter puberty, because this timing has been linked with the risk of developing health conditions ? such as obesity, diabetes, heart disease and breast cancer ? later in life. To find the genes that influence puberty in girls, researchers analyzed the genomes of more than 180,000 women of European descent, and found more than 100 genetic variations that were associated with the age at which girls experienced their first menstrual period. Six of these genetic differences were found within a set of genes known as "imprinted" genes, according to the study, published today (July 23) in the journal Nature.