Background: Little is known about reproductive outcome risks for Vietnamese women delivering infants and fetuses in the U.S.

Methods: Using data from a large population-based registry, we explored risks of selected congenital malformation phenotypes in offspring of Vietnamese women in California. Data were derived from the California Birth Defects Monitoring Program, a population-based active surveillance system for collecting information on infants and fetuses with congenital malformations using multiple source ascertainment. Approximately 3.4 million births (liveborn and stillborn) occurred during the ascertainment period, 1985-97. Information on maternal race/ethnic background was obtained from California birth certificate and fetal death files. Vietnamese women delivered 45,453 births and 1,257,853 births were delivered to non-Hispanic white women.

Results: The overall prevalence of structural congenital malformations was 1.92 among Vietnamese and 2.63 among non-Hispanic whites per 100 births and fetal deaths. Grouping by 20 3-digit malformation codes of the International Classification of Diseases-Ninth Revision revealed relative risks of 0.8 or less for spina bifida, eye, upper alimentary, genital, urinary, musculoskeletal, "other" limb, and "other" musculoskeletal anomalies, and relative risks of 1.3 or more for anencephaly and chromosomal anomalies. Grouping by the more specific 4-digit malformation codes revealed 50, among 178, malformation groupings with associated relative risks of >or=1.3 or <or=0.8. Seventeen relative risk estimates suggested elevated risks for Vietnamese births, and 33 estimates suggested lowered risks for Vietnamese, relative to non-Hispanic white births. Simultaneous adjustment for maternal age, plurality, and child sex, did not substantially alter observed risks.

Conclusions: These data contribute to a relatively sparse literature about an important subgroup of the U.S. population, but can not elucidate whether increased or decreased risks among Vietnamese are related to potential underlying genetic susceptibilities, to cultural, social, or political differences that could modify exposures, or to the many potential combinations between susceptibilities and exposures.