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STANFORD, Calif., Oct. 1 -- The risk for some pregnancy complications may be greater when one partner is white and the other Asian, researchers found.

Gestational diabetes rates were higher among Asian-white couples than among white couples regardless of which parent was Asian (adjusted odds ratio 2.4 to 2.6), found Yasser El-Sayed, M.D., of Stanford University Medical Center here, and colleagues in the October issue of the American Journal of Obstetrics and Gynecology.

In the retrospective study, though, Caesarean delivery was least common for white-mother/Asian-father couples but most common when the mother was Asian and the father white (23% versus 33.2%).

These findings may help in counseling parents and planning prenatal care for the approximately 14.3% of the U.S. population born with mixed Asian and white ancestry, the researchers said.

They also said the results give a unique window into the interaction of genetics and environment on perinatal complications.

Most research on pregnancy outcomes among interracial couples has focused on African-American and white couples, the researchers noted.

And it's been unclear whether ethnicity is simply a marker for socioeconomic status or other factors, said Aaron B. Caughey, M.D., M.P.H., Ph.D., of the University of California San Francisco and a member of the research team.

To examine those issues, the researchers retrospectively reviewed outcomes for all white, Asian, and interracial Asian-white couples that delivered babies at the Lucile Packard Children's Hospital at Stanford from 2000 through 2005.

The cohort included 868 Asian-white, 3,226 Asian, and 5,575 white couples living in a single geographic area, which Dr. Caughey said made the population more homogeneous for socioeconomic status and other factors.

During pregnancy, gestational diabetes incidence was higher for Asian women than white women as expected from previous studies (5.73% versus 1.61%, adjusted OR 4.7, 95% confidence interval 3.6 to 6.2).

This difference has been attributed to an underlying genetic predisposition to gestational diabetes among individuals of Asian descent given the low overall obesity rates in this population, the researchers noted.

Notably, though, interracial couples had intermediate risk regardless of whether the mother was Asian or white (3.91% incidence for Asian-mother/white-father and 3.37% for white-mother/Asian-father, P<0.001 versus white couples).

The association remained unchanged with adjustment for maternal age, education, and insurance status as well as number of prenatal visits.

These findings suggest a significant paternal contribution, the researchers said.

Whereas gestational diabetes has been typically taken as a sign that the mother is at risk for diabetes later in life, they wrote, "future research should investigate … whether the fathers in these cases are also at increased risk of developing type 2 diabetes mellitus."

Although the gestational age at delivery was clinically similar between groups, average birth weights were lowest for Asian couples (7.07 lb), highest for Caucasian couples (7.49 lb), and intermediate for mixed couples (7.40 lb for Asian-mother/white-father and 7.32 lb for white-mother/Asian-father, P<0.001).

Macrosomia, defined as infants over 4,000 g, showed a similar incidence pattern with low risk among Asian couples (OR 0.35, 95% CI 0.29 to 0.43).

Interracial couples were at intermediate risk of having a baby who was large for gestational age, but the difference was of only borderline significance for Asian-mother/white-father couples and not significant for white-mother/Asian-father couples compared with white couples (OR 0.78, 95% CI 0.59 to 1.0, and OR 0.68, 95% CI 0.40 to1.2).

For Caesarean delivery, though, risk consistently peaked among Asian-mother/white-father couples. The odds were 30% to 70% higher than for white couples whether all deliveries or only term, nulliparous, labored deliveries were considered (P<0.001).

Caesarean rates were:

• 26% for Asian couples
• 33.2% for Asian-mother/white-father couples
• 23% for white-mother/Asian-father couples
• 28.7% for white couples

Since the average birth weights were similar for both types of interracial couples, the difference may have been that a smaller average pelvis size among Asian women was "less able to accommodate the slightly larger average birth weights encountered by interracial couples," the researchers suggested.

Rates of preterm delivery and hypertensive disorders of pregnancy were similar among groups, but the researchers noted that the study was underpowered for both outcomes.

"Interracial couples are still relatively rare, making it difficult to amass a sufficient population to have an adequately powered study of each subgroup's interracial outcomes," they noted.

Another limitation was that the broadly defined racial categories could not adequately reflect the genetic diversity of these groups, they said.

The researchers reported no conflicts of interest.

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