Introduction Unlike other chronic inflammatory diseases, little is known about how ankylosing spondylitis (AS) affects outcomes of pregnancy and birth characteristics.

Methods In a nationwide population-based case–control study, 388 deliveries among women with AS (identified in the Swedish National Patient Register and Medical Birth Register) and deliveries among matched controls (n=1082) from the general population were included. Information regarding pregnancies after AS diagnosis, birth outcomes and possible confounders were retrieved from the national Swedish registers. ORs with 95% CIs were calculated with generalised estimating equations.

Results Emergency and elective Caesarean section (CS) were performed in 16.5% and 9.8% of deliveries among women with AS compared with 6.5% and 6.9%, respectively, in population controls, resulting in OR of 3.00 (95% CI 2.01 to 4.46) and 1.66 (95% CI 1.09 to 2.54), respectively. Offspring of women with AS were more often preterm (9.0% vs 4.9%) and small-for-gestational-age (SGA) (3.1% vs 1.5%), resulting in an OR of 1.92 (95% CI 1.17 to 3.15) and 2.12 (95% CI 1.00 to 4.50), respectively. Adjustment for smoking habits, age, educational level, parity and exclusion of women with comorbidities resulted in similar or only slightly lower point estimates of risk. Cases with a more extensive antirheumatic therapy exposures tended to have a higher risk for elective CS and being SGA.

Conclusions Women with AS had a higher prevalence for several adverse birth outcomes, with results suggesting an influence by both disease severity and comorbidities.