The charts that doctors use when assessing a baby's birth weight should take into account the mother's ethnicity, to better predict health problems after birth, a new study from Canada suggests.

For example, charts may misclassify Asian women's babies as too small, or miss noting that some bigger babies are at risk for complications, according to a study published today (Nov. 11) in the Canadian Medical Association Journal.

As a result, some babies may face unneeded scrutiny of their health, and even lengthier hospital stays, said study co-author Marcelo Urquia, an epidemiologist at the Centre for Research on Inner City Health of St. Michael's Hospital in Toronto. [11 Big Fat Pregnancy Myths]

Measuring up

Birth weight charts, which plot the weight of babies in a certain region, are used to identify babies at higher risk of complications after birth.

"Those babies who are at the extremes are considered to be small or large for gestational age," Urquia said.

Birth weight charts are important because babies who are truly "small for gestational age," meaning they are smaller than about 90 percent of their peers, may have faced growth restriction in the womb. Compared to their normal-weight peers, these babies are likelier to face a lengthy stay in the neonatal intensive care unit (NICU), have breathing troubles and die.

But because of birth weight charts that don't take ethnicity into account, some babies may mistakenly be considered small for gestational age.

On the other end of the spectrum, big babies may face complications if their shoulders don't fit through the mother's birth canal; these complications can include broken collarbones and damaged nerves in the babies' necks. Their mothers are also likelier to face tearing and excessive bleeding during the birth, and babies born to women with gestational diabetes are at higher risk of low blood sugar.

But some low-birth-weight babies are just constitutionally small, and some big babies have very tall parents, so it's important to tease out whether a baby's weight reflects underlying health issues or simply genetics, Urquia said.

Unrepresentative charts

In the new study, Urquia and his colleagues looked at data from more than 1 million singleton births in Ontario, Canada from 2002 to 2012. About a third of those births were to people who had immigrated to Canada, the majority of whom were from South Asia or East Asia.

Canadian birth weight charts, which are based mostly on babies of largely Western European ancestry, identified about 16 percent of the babies of Asian immigrants as small for gestational age, but 6 percent of those babies would not have been considered small compared to other babies from their mother's countries of origin.

Babies who would be considered small in the mother's country of origin were likelier to die or face a lengthy NICU stay, but that wasn't the case for babies who were only considered small on the Canadian growth charts. On average, the babies of Asian immigrants tend to be about 0.4 lbs. (0.2 kilograms) lighter than their peers in Canada, Urquia said.

On the flipside, the Canadian charts missed about 4 percent of the babies who would have been considered large for gestational age in their mother's home countries. Women who gave birth to these babies were indeed likelier to experience tearing and excessive bleeding after the delivery, the study found.

Accurately identifying babies at risk of health complications is important, because hospitals often use a baby's weight as a factor in deciding whether to let a baby's parents take the child home, or keep the baby in the NICU for observation.

Unnecessarily keeping a baby in the hospital can cause problems because long hospital stays can interfere with breastfeeding and with mother-baby bonding, Urquia said. In addition, "the parents will be stressed out because they think something may be wrong with the baby," Urquia told Live Science.

The findings suggest that doctors should refer to the growth charts specific to a person's country of origin when trying to predict health outcomes, particularly for people from Asia. Although the study was conducted in Canada, a study last year found similar results hold in the United States, Urquia said.

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