No matter when it happens, puberty is no fun. But for girls, entering womanhood earlier or later than average can mean more than just the social awkwardness of being quick to mature or a late bloomer. It can have important health consequences later in life.
In general, girls in the developed world are maturing faster now than ever before. In the mid-nineteenth century the age at menarche (the first menstrual period, a milestone usually reached about two to three years after puberty begins) was about 16, but by the end of the twentieth century that number had fallen to about 13. Earlier age at menarche has been associated with an increased risk for breast, ovarian and endometrial cancer. On the other hand, later menarche is linked to increased risk for osteoporosis and cardiovascular disease.
Much of the change in the rate of maturation for girls has been attributed to better nutrition and rising rates of obesity, but new research shows that the age at menarche is also influenced by genetics.
Four papers (Ong et al., Sulem et al., Perry et al., and He et al.), published online yesterday in the journal Nature Genetics, analyzed data from a total of more than 60,000 women with European ancestry and identified two regions of the genome that contain variations associated with small differences in age at menarche. One of these four studies, and a fifth paper (Stolk et al.), also found variations associated with the timing of menopause, the other end of the reproductive spectrum for women.
All four studies looking at age of menarche found a connection with a region of chromosome 6. Each paper identified slightly different variants, but they all appear to be zeroing in on a single, yet to be found, causative variation in the DNA. As an example of the effect of the chromosome 6 SNPs, Ong et al. found that compared to those with two Cs, each T at meant that woman got her first period about six weeks earlier.
Each T at was also associated with 1.20 times greater odds of breast development at age 10, 1.26 times greater odds of advanced breast development between the ages of 9 and 16 and a faster rate of increase in height and weight.
Although girls who go through puberty earlier than their peers are tall for their age during childhood, their growth stops sooner and they are generally shorter as adults. Ong et al. found that each T at was associated with a reduction in adult height of about 0.15 inches.
Two studies also found an association between variations on chromosome 9 with age at menarche. Each C at (the most significant SNP found in He et al.), for example, translated into about five weeks earlier menarche.
He et al. and Stolk et al. found variations in five regions of the genome associated with another big change in a woman’s life: menopause. The risks associated with the timing of menopause are the reverse of those associated with menarche: later menopause has been associated with higher risk for certain cancers and earlier menopause is linked to increased risk for osteoporosis and cardiovascular disease.
Finally, all of you boys and men out there need not feel left out: Ong et al. also looked at the effects of on puberty in males. They found that each copy of an T translated into 1.26 greater odds of advanced voice breaking at age 15, 1.19 times greater odds of advanced pubic hair at age 13, faster rate of growth at age 10 and about a tenth of an inch less in adult height.