OBJECTIVERoux-en-Y gastric bypass (RYGB) and pregnancy markedly alter glucose metabolism, but evidence on glucose metabolism in pregnancy after RYGB is limited. Thus, the aims of the Bariatric Surgery and Consequences for Mother and Baby in Pregnancy study were to investigate interstitial glucose (IG) profiles during pregnancy, risk factors associated with hypoglycemia, and the association between fetal growth and hypoglycemia in pregnant women previously treated with RYGB, compared with control participants.RESEARCH DESIGN AND METHODSTwenty-three pregnant women with RYGB and 23 BMI- and parity-matched pregnant women (control group) were prospectively studied with continuous glucose monitoring in their first, second, and third trimesters, and 4 weeks postpartum. Time in range (TIR) was defined as time with an IG level of 3.5–7.8 mmol/L.RESULTSWomen with RYGB were 4 years (interquartile range [IQR] 0–7) older than control participants. Pregnancies occurred 30 months (IQR 15–98) after RYGB, which induced a reduction in BMI from 45 kg/m2 (IQR 42–54) presurgery to 32 kg/m2 (IQR 27–39) prepregnancy. Women with RYGB spent decreased TIR (87.3–89.5% vs. 93.3–96.1%; P < 0.01) owing to an approximately twofold increased time above range and increased time below range (TBR) throughout pregnancy and postpartum compared with control participants. Women with increased TBR had a longer surgery-to-conception interval, lower nadir weight, and greater weight loss after RYGB. Finally, women giving birth to small-for-gestational age neonates experienced slightly increased TBR.CONCLUSIONSWomen with RYGB were more exposed to hypoglycemia and hyperglycemia during pregnancy compared with control participants. Further research should investigate whether hypoglycemia during pregnancy in women with RYGB is associated with decreased fetal growth.