Non‐obstetric abdominal surgery during pregnancy and birth outcomes: A Danish registry‐based cohort study

INTRODUCTION: Surgery during pregnancy may increase the risk of adverse birth outcomes. In this nationwide registry-based cohort study including women aged 15-54 years with singleton birth or miscarriage, we examined the association between non-obstetric abdominal surgery during pregnancy and the birth outcomes small-for-gestational-age (SGA), preterm birth, and miscarriage. MATERIAL AND METHODS: The study used data on births or miscarriages from the large national Danish registries in 1997-2015. We calculated absolute risks and risk differences for the main outcomes and used Cox regression analysis with non-obstetric abdominal surgery as a time-varying exposure, adjusting for maternal age, year of last menstrual period, major abdominal surgery before pregnancy, maternal smoking status, rheumatoid arthritis, diabetes and inflammatory bowel disease. Our main outcome measures were risks and hazard ratios (HRs) for SGA, very preterm or preterm birth, and miscarriage after gestational week 7 overall, stratified by calendar year, and, for SGA, trimester of pregnancy. Finally, absolute risk of miscarriage stratified by time since surgery. RESULTS: Absolute risks in surgically treated vs untreated were 3.4% vs 2.7% for SGA (adjusted HR 1.3, 95% CI 1.1-1.5), 2.2% vs 0.8% for very preterm birth (adjusted HR 2.8, 95% CI 2.2-3.5), 8.3% vs 4.3% for preterm birth (adjusted HR 2.1, 95% CI 1.9-2.3), and 8.2% vs 6.1% for miscarriage (adjusted HR 3.1, 95% CI 2.7-3.5). For miscarriage, the risk was highest the first week after surgery and levelled out after 2 weeks. CONCLUSIONS: Surgery during pregnancy is associated with an increased risk of SGA, very preterm birth, preterm birth and miscarriage, and the risk of miscarriage is highest the first week after surgery.