Antenatal Zika and SGA risk

Antenatal Zika virus infection is associated with numerous birth defects, and new research shows that it significantly increases risk of giving birth to a small-for-gestational-age (SGA) baby. Such neonates are at a higher risk of morbidity and mortality in infancy and early childhood, and of chronic disease in later life.

Methods
The retrospective observational study, published in Obstetrics & Gynecology, analyzed New York City birth record data to evaluate associations between antenatal Zika virus and low birth weight, SGA, and preterm birth (PTB). Although New York City has not detected local mosquito-borne transmission of the virus, more than 1,000 imported Zika infections were diagnosed there in 2016 as a result of the outbreak in the Americas. 

Birth record data for women who delivered liveborn singleton neonates in 2016 were linked to data for pregnant women with Zika infection reported to the New York City Health Department. The authors restricted the analysis to nonsmoking, nonwhite women and adjusted for maternal characteristics. They also used modified Poisson regression to assess risk of having an SGA neonate and of delivering preterm among infected women. 

Findings
In 2016, a total of 116,034 women gave birth to a singleton neonate in New York City. Of them, 251 (0.2%) had antenatal Zika virus infection. A total of 74,955 (64.6%) women remained for analysis after exclusions. In this cohort, a higher proportion of Zika-infected women were in their first pregnancy, aged younger than 20 years, identified as non-Hispanic black, and were born outside of the United States. 

Twenty-eight (11.2%) women infected with and 4,340 women (5.8%) without Zika virus gave birth to an SGA neonate. After adjustment, risk of having an SGA neonate was 1.8 times higher for women with antenatal Zika virus infection (95% CI 1.3-2.6). For women with Zika virus infection, prevalence of PTB was 8.8%; for women without infection, prevalence was 7.8%. The authors found no association between antenatal Zika virus infection and PTB in the adjusted model (relative risk 1.0; 95% CI 0.69-1.6). 

Mean birth weight of the 228 neonates born at term to women with antenatal Zika virus infection was 3,256±479 g vs 3,303±477 g for the 68,861 term neonates born to women without Zika virus infection. The difference in birth weight was not significant in crude or adjusted analyses. 

Conclusions
The authors believe that for this cohort, antenatal Zika virus was associated with an increased risk of having an SGA neonate, but not of PTB or lower mean birth weight of term neonates. However, prospective studies of women with Zika virus infection during pregnancy are needed to validate the findings.