Children born to women in their 30s and 40s appear significantly more likely to be left-handed than those born to younger women, a Canadian researcher reported last week. A study of 2,228 college freshmen found those whose mothers were ages 40 and above when they were born had more than twice the rate of left-handedness as those whose mothers were between 17 and 29, reported Stanley Coren of the University of British Columbia in the New England Journal of Medicine.
Compared to the students with younger mothers, Coren found an 11% increase in left-handedness among those born to mothers aged 25 to 29, a 25% increase in those born to mothers aged 30 to 34, and a 69% increase in those born to mothers aged 35 and 39. Moreover, students whose mothers were 40 and above had a 128% increase.
Coren said the findings coincide with previous research suggesting that babies subjected to excess stress during birth appear more likely to be left-handed. Common sources of stress include prolonged labor, an interruption of the baby's oxygen supply during birth, premature birth and breech birth. Older mothers are more likely to have more stressful deliveries.