A new study offers some intersting insight into how having a male twin could affect a girl's weight gain and overall health.
The study, published in the International Journal of Obesity, researched a database of Swedish twins born between 1886 and 1958. They looked closely at the 17,500 females born in the period of 1998 through 2003 who had a twin. Just under half of these girls had a male twin, while the others had a female fraternal twin.
They conducted phone interviews with the girls — now women — who were between the ages of 42 and 103 years old to determine their height and weight, whether or not they had diabetes, high cholesterol and other health realted information.
They found that the women who had a male twin at birth gained more weight over the years than women who had a female twin. The difference was really just a few pounds, but it was statistically significant enough to show a marked difference. Women with a male twin were also more likely to have excess fats and cholesterol.
Why the difference? Researchers think that females that share the womb with males may be exposed to greater amounts of predominantly male hormones, called androgens, than those without a male twin. (This could also explain why other studies have shown that females in boy girl twin pairs are more aggressive than those with a female twin.)
Of course, it could also just be the difference between growing up with a brother — think larger portions at mealtime — and growing up with a sister, regardless of the twin factor.