"Below-average" birth weights are normal for babies with Asian and South Asian dads, regardless of the mom's ethnic origin, a new study suggests.

In fact, the father's ethnic origin appears to have a bigger effect on the baby's birth weight than the mother's ethnic origin, suggests the study by researchers at the Institute for Clinical Evaluative Sciences and St. Michael's Hospital in Toronto. The study was based on data on birth weights and the parents' countries of origin, which is strongly linked to ethnicity for the Asian and South Asian countries studied.

The researchers hope this new knowledge will help ease worry — and potential overfeeding of babies and children — by concerned parents and grandparents of babies who are a healthy size for their genetic background, but appear small on Canadian growth charts, which are largely based on Caucasian babies.

Parents don't want their kids to be small. - Dr. Joel Ray, St. Michael's Hospital

"Mixed unions are now becoming extremely commonplace in Canada," noted Dr. Joel Ray, a co-author of the research paper published this week in the Journal of Epidemiology and Community Health.

Babies are weighed at birth partly to screen for possible health problems, said Ray, a researcher in the Department of Obstetrics and Gynecology at the University of Toronto and St. Michael's Hospital. Those who are considered too small may face further medical tests to check for such problems, causing stress and worry for their families.

He added that previous studies have shown that babies' average birth weights are linked to their ethnicity, and that Asian and South Asian babies are often smaller than the Canadian average. Doctors have started taking the ethnicity of the mother into account when interpreting a baby's birth weight to figure out whether the child is healthy or not.

But the new study, based on data about birth weights and the country of origin for the babies' parents from more than one million live births in Ontario, suggests that the father's country of origin (and therefore ethnicity) has a bigger effect on the baby's birth weight than the mother's country of origin (and ethnicity). The study examined the birth weights of children born to parents when one or both of them were from Canada, Bangladesh, Sri Lanka, Pakistan, India, the Philippines, Vietnam, Korea, Hong Kong or China.

Baby boys born to two Canadian parents average 3,530 grams or 7 pounds, 12.5 ounces. Those born to a Canadian father and an Asian or South Asian mother averaged 3,362 grams or 7 pounds 6.6 ounces; (168 grams or 4.8 per cent less). And those born to a Canadian mother and an Asian or South Asian father averaged 3,294 grams or 7 pounds, 4.2 ounces (236 grams or 6.7 per cent less).

Those with two Asian or South Asian parents were the smallest, averaging 3,279 grams or 7 pounds, 3.5 ounces. The trends were similar, but less pronounced for baby girls.

Overfeeding risk

Ray said it's important for parents and doctors to recognize this when a child's weight is plotted on Canadian growth charts.

"They're taking it pretty seriously. Parents don't want their kids to be small," Ray said, adding that that's true regardless of the culture.

"It's very important that we don't create any unnecessary pause or worry for those parents… Parents may overreact by overfeeding."

He said medical professionals have anecdotal evidence that that is happening in some ethnic communities when a child's size is below average compared to the Canadian average.

"They look too small and they're not. They're normal. They just simply need to be left alone — normal sized portions at meals."

Doctors worry that overfeeding a child may lead to problems such as obesity later in life.

The researchers are now looking at whether children's growth curves also differ depending on their father's ethnic origin, and whether to incorporate that information into standard growth charts.