Impact of Maternal BMI on Rubella Nonimmunity at Antenatal Screening

The aim of this study was to examine the relationship between BMI and the incidence of rubella nonimmunity among pregnant women with regard to the World Health Organization categories.

This was a retrospective cohort study of 117,063 eligible gravidas, managed between 1998 and 2015 in a university hospital of Hong Kong, China. BMI at antenatal booking was banded using the following World Health Organization definitions: < 18.5 kg/m² , 18.5 to 24.9 kg/m² , 25.0 to 29.9 kg/m² , and ≥30.0 kg/m² . Maternal rubella nonimmunity status by BMI was assessed. Incidence rates were also assessed after adjusting for maternal advanced age, short stature < 151 cm, mothers' birthplace, and postobstetric history covariates.

Rubella nonimmunity incidence increased as the BMI increased (P < 0.001). Gravidas with high BMI were more likely to be nonimmune if born in Hong Kong (odds ratio [OR], 1.234; 95% CI: 1.159-1.315; P < 0.001) compared with those born outside of Hong Kong (OR, 1.066; 95% CI: 0.997-1.141; P = 0.063). After adjusting for covariates, women with BMI ≥ 25 kg/m² had 1.127 (P < 0.001; 95% CI: 1.074-1.182) greater odds of being nonimmune.

High maternal BMI (≥ 25.0 kg/m² ) is associated with reduced rubella immunity, an effect confined to gravidas with almost complete vaccine coverage in childhood.