Why are some children taller than others? It seems their height might be linked at least partly to how old - or young - their mom was when she got her first menstrual period.
A new study suggests that the earlier a woman reached this milestone of puberty, called menarche, the slightly faster her kids will grow.
The research team looked at the growth of more than 30,000 boys and girls born in the U.S. between 1959 and 1966.
From these kids, they selected two groups who were born to women who reached menarche at relative extremes: before age 12 and after age 14. (Most girls tend to have their first period between the ages of 12 and 13.)
From birth through age 8, the two groups had small but consistent differences in height, weight and body mass index (BMI) — a measure of weight that accounts for height.
By age 8, children of mothers who had early periods were an average of a third of an inch taller and 2 pounds heavier than children whose mothers' periods started late. Their average BMI was also slightly higher, although still within the normal range. (A third of an inch may not sound like much, but as an average in a group of this size it's a significant difference, according to the research team.)
The differences held regardless of sex, race, socioeconomic status, and mother's age and size, the researchers say in the International Journal of Obesity.
"Growth is a complex phenomenon influenced by both genetic and environmental factors," lead researcher Dr. Olga Basso of McGill University in Montreal, Canada, told Reuters Health by e-mail. A mother's age at her first period, which is partly influenced by genes, is probably just one factor in this process, she said.
Her findings confirm those from a recent British study led by Dr. Ken Ong of Addenbrooke's Hospital in Cambridge.
"Asking about age at menarche in the mother may provide a clue as to why some infants grow faster than others," Ong told Reuters Health by e-mail.
He also pointed out that heavier girls tend to get their periods earlier; these girls are at increased risks of long-term diseases such as diabetes and heart disease. More studies are needed to find ways to counteract these risks, Ong said.
The researchers don't have a solid explanation for their findings. The fact that a mother's age at her first period affected both boys and girls means the effect is probably related to genes that program overall growth, rather than specifically female growth, they said.
"Other factors that we could not account for, such as the mother's dietary habits that she transmits to her children, could have caused both early menarche in the mother and the larger size of her children," Basso said. "I think that our findings underscore the complexity underlying the growth process."