The groups differed in several maternal characteristics, including age, smoking, race or ethnicity, maternal education, BMI before pregnancy, first pregnancy, gestational diabetes, and study centre (table 1).
Table 1 Characteristics of case and control mothers participating in national birth defects prevention study, 1997-2003. Figures are numbers (percentages) of women
Overall, 16% (n=81) of case mothers and 9% (n=425) of control mothers reported a genitourinary infection; 4% (n=21) and 2% (n=98) reported a sexually transmitted infection and 13% (n=67) and 7% (n=338) reported a urinary tract infection, respectively.
The figure shows that younger mothers (under 25) tended to report higher rates of genitourinary infection than mothers aged 25 and over. This difference was particularly noticeable for the combination of infections (no case mothers aged 25 and over reported both a sexually transmitted infection and a urinary tract infection).
Prevalence of genitourinary infection (per 1000) among case mothers and control mothers, stratified by maternal age, national birth defects prevention study, 1997-2003 (STI=sexually transmitted infection, UTI=urinary tract infection)
Table 2 shows odds ratios for gastroschisis by exposure group, first stratified by maternal age and then pooled to examine maternal age specific estimates. The variations were not significantly different with Breslow-Day or logit plot assessment. Crude odds ratios were higher: 2.0 (95% confidence interval 1.6 to 2.6) for sexually transmitted infection or urinary tract infections; 1.7 (1.0 to 3.0) for sexually transmitted infection only; 1.9 (1.5 to 2.6) for urinary tract infection only; and 6.8 (2.6 to 17.5) for sexually transmitted infection and genitourinary infection.
Table 2 Crude odds ratios with 95% confidence intervals for risk of gastroschisis according to genitourinary infection and stratified by maternal age, national birth defects prevention study, 1997-2003
Table 3 shows a moderately increased risk for gastroschisis with exposure to genitourinary infection, particularly for the combination of infections, after adjustment for potential confounders. The risk also seemed relatively high for gastroschisis associated with other malformations (multiples), although the confidence intervals are wide and overlap those among isolated cases.
Table 3 Adjusted odds ratios with 95% confidence intervals for isolated, multiple, and all cases of gastroschisis according to genitourinary tract infection, national birth defects prevention study, 1997-2003
For most exposures, the pathogen was not documented. Among women who reported a urinary tract infection plus a sexually transmitted infection, the most common pathogen, Chlamydia, was reported by 18% of mothers of control infants and 43% of case mothers. Results were unchanged when we excluded case mothers who reported human papillomavirus infection (data not shown). We also examined any association with fever or antibiotic use. To the extent that such information was available, fever or antibiotic use for genitourinary infection was not associated with an increased risk of gastroschisis.