Risk of pre-eclampsia after gastric bypass: a matched cohort study.

Objective: To investigate whether gastric bypass before pregnancy is associated with reduced risk of pre-eclampsia.

Design: Nationwide matched cohort study. Setting: Swedish national health care. Population: A total of 843 667 singleton pregnancies without pre-pregnancy hypertension were identified in the Swedish Medical Birth Register between 2007 and 2014, of which 2930 had a history of gastric bypass and a pre-surgery weight available from the Scandinavian Obesity Surgery Registry. Two matched control groups (pre-surgery and early-pregnancy body mass index [BMI]) were propensity score matched separately for nulliparous and parous births, to post-gastric bypass pregnancies (npre-surgery-BMI  = 2634:2634/nearly-pregnancy-BMI  = 2766:2766) on pre-surgery/early-pregnancy BMI, diabetes status (pre-surgery/pre-conception), maternal age, early-pregnancy smoking status, educational level, height, country of birth, delivery year and history of pre-eclampsia. Main outcome measures: Pre-eclampsia categorised into any, preterm onset (<37+0  weeks) and term onset (≥37+0  weeks).

Results: In post-gastric bypass pregnancies, mean pre-surgery BMI was 42.9 kg/m2 and mean BMI loss between surgery and early pregnancy was 14.0 kg/m2 (39 kg). Post-gastric bypass pregnancies had lower risk of pre-eclampsia compared with pre-surgery BMI-matched controls (1.7 versus 9.7 per 100 pregnancies; hazard ratio [HR] 0.21, 95% CI 0.15-0.28) and early-pregnancy BMI-matched controls (1.9 versus 5.0 per 100 pregnancies; HR 0.44, 95% CI 0.33-0.60). Although relative risks for pre-eclampsia for post-gastric bypass pregnancies versus pre-surgery matched controls was similar, absolute risk differences (RD) were significantly greater for nulliparous women (RD -13.6 per 100 pregnancies, 95% CI -16.1 to -11.2) versus parous women (RD -4.4 per 100 pregnancies, 95% CI -5.7 to -3.1).

Conclusion: We found that gastric bypass was associated with lower risk of pre-eclampsia, with the largest absolute risk reduction among nulliparous women. Tweetable abstract: In this large study including two comparison groups matched for pre-surgery or early-pregnancy BMI, gastric bypass was associated with lower risk of pre-eclampsia.