Some babies born to immigrant parents are incorrectly classified as underweight — which could lead to unnecessary tests and stress — but new birth weight charts could avoid the problem, Canadian doctors say.

When a baby is born, the first questions the new parents are often asked is "Girl or boy?" Then, "How much did the baby weigh?"

The answer from some parents of South Asian background is often different than that of other parents. But a lower weight could mean that doctors think the baby is at higher risk of developmental issues. That leads to more blood tests, longer hospital stays and closer followup when the baby isn't actually at risk because its weight is within the normal range for its ethnic group, researchers say.

In Wednesday's issue of the Journal of Obstetrics and Gynecology Canada, Dr. Joel Ray and his team published new birth weight curves based on the ethnicity of the baby's parents. They used birth registry information for 760,000 live births in Ontario.

Until now, birth weight curves for Canada were outdated and risked misclassifying some South Asian and East Asian babies as abnormally small.

"About one in 10 male infants of South Asian ancestry would be classified as small for gestational age, meaning under the 10th percentile of birth weight, if you used a conventional Canadian curve rather than their own ethnicity-specific curve," Ray said.

It's about the same for South Asian females and about one in 18 for East Asian males from countries such as China or Korea, he added.

Babies classified as small for gestational age could be scrutinized more closely with blood tests in hospital and then checked more often after going home — a time when all new parents are adjusting to their new role and don't need unnecessary stress.

Immigrants' newborns up to 250 grams lighter

Newborns of immigrant mothers weighed up to 250 grams less at birth than those of Canadian-born women, with the exception of those originating from European and Western nations, the study's authors said.

"It would be nice to have a South Asian growth chart," said new mom Manisha Mehta in Toronto. Her son was on the 15th percentile when he was born, meaning 85 per cent of baby boys were bigger than him on conventional charts.

"That way parents can feel OK knowing that their child is growing and is healthy."

The babies who weigh small on the old charts don't typically have disease in the womb or after birth, so the ethnic differences likely reflect differences in genetic programming, Ray said.

By using the new curves, doctors, midwives and nurses could prevent misidentifying otherwise healthy newborns as small for gestational age or missing those who are truly large for gestational age and should be delivered by elective cesarean section, the researchers said.

The investigators also hope to chart the growth of newborns and children from different ethnic groups.

The study was funded by the Canadian Foundation for the Study of Infant Deaths.

With files from CBC's Pauline Dakin