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Researchers have found that Asian women pregnant with white men's babies are more likely to deliver by cesarean section, and are now recommending that doctors factor in both a mother and father's race when they consider the risk of birth complications.

Thirty-three per cent of couples consisting of an Asian mother and white father had C-sections. By comparison, 23 per cent of couples with a white mother and Asian father had cesareans. The researchers speculate this is because Asian women tend to be "smaller people" than Caucasians, and that their smaller pelvises cannot accommodate a more sizable baby, in some cases.

"We know that one of the best predictors of cesarean delivery is maternal height," said Aaron Caughey, a University of California, San Francisco doctor who co-authored the study, released today in the American Journal of Obstetrics and Gynecology.

"If you have a smaller person, they're more likely to have a cesarean delivery, given the same size baby. Since we know there is a paternal contribution to fetal size, if you then have a bigger father you're more likely to have a cesarean delivery."

Co-author Yasser El-Sayed, an associate professor of obstetrics and gynecology at the Stanford University School of Medicine, said "a physician managing [the labour of someone]who fits into one of these categories may use this information if the labour is dysfunctional, if the labour's quite protracted. It may inform some of their decision-making."

Although controversial, race-based surveys of this kind will proliferate as prenatal doctors work with increasingly diverse patients, Dr. El-Sayed said.The doctors looked at data from 5,575 white, 3,226 Asian and 868 Asian-white couples who delivered between 2000 and 2005 at Lucile Packard Children's Hospital in Palo Alto, Calif., where Dr. El-Sayed works as an obstetrician. For the purpose of the study, all people who reported Chinese, Filipino, Japanese, Korean, Vietnamese, Indonesian, Taiwanese, Indian, Hmong, Thai or Cambodian as their ethnicity were categorized as Asian.

The doctors did not separate first- and second-generation Asians and do not know whether the C-section disparities will disappear among children of mixed racial marriages.

"We know dietary changes, lifestyle changes, all kinds of changes occur as an immigrant population spends time in a new country," says Dr. Caughey, who also suggested some differences might persist because of genetics.

"I think what we're going to see increasingly over the next decade is a transition from race- and ethnicity-only studies to race, ethnicity and genetic studies."

Few studies have focused specifically on Asian-Caucasian couples. Most research has focused on interracial Caucasian and black couples, and those studies found that couples with a white father and black mother had notably high rates of stillbirths, pre-term births and low-birth-weight babies, Dr. Caughey said.

Hani Akoury, an obstetrician specializing in high-risk pregnancies and births at Sunnybrook Health Sciences Centre in Toronto, was critical of the study. He said surveys of this kind might put undue stress on interracial parents.

"If you come to me in my office and you're Asian and you married a white man, and I tell you, 'Oh, by the way, before you leave: You may have a higher cesarean-section rate than other women and I'll do my best to help you,' ... it's going to be impractical in my view, that information. I would rather not even hear it because I'm going to cause more anxiety in a situation where I can't influence much," Dr. Akoury said.

Noting that "it's not meant to be controversial," Dr. El-Sayed warned clinicians not to use his study in "any categorical way." At the same time, he suggested the findings could be used to "refine" clinical management.