In our study, pregnancy after bariatric surgery was associated with a lower incidence of gestational diabetes. These findings are consistent with previous studies [10, 24,25,26,27] and could be attributable to two main changes that occur after bariatric surgery: the first being weight loss, as obesity is a well-known risk factor for the development of gestational diabetes, and the second the changes that occur in glucose metabolism after bariatric surgery, independently of weight loss [11]. Previous studies have shown that bariatric surgery leads to improved fasting glucose and exaggerated postprandial insulin response, which in some cases leads to postprandial hypoglycemia, and that these changes in glucose metabolism persist during pregnancy [28,29,30].

Additionally, among patients with gestational diabetes, previous bariatric surgery was associated with a diagnosis later in pregnancy, implying that these patients were exposed to hyperglycemia for a shorter period. To the authors’ knowledge, there is limited existing evidence regarding the effect of bariatric surgery on the timing of diagnosis of gestational diabetes, and the results in our study could have been influenced by the different diagnostic methods used in the second trimester in the two groups.

In our study, newborns of postbariatric pregnancies were smaller; there was a higher risk of fetal growth restriction and lower risk of fetal macrosomia. Likewise, previous studies have shown that pregnancy after bariatric surgery is associated with a lower risk of LGA newborns, and higher risk of SGA newborns and fetal growth restriction [10, 12, 24, 27, 31]. Similarly, weight loss and glucose metabolism changes produced by bariatric surgery seem to play a major role in determining newborn size [11, 28, 32]. Other proposed mechanisms include maternal deficiency in micronutrients and protein and changes in inflammatory cytokines and cellular oxidative stress [11, 33]. Low cord blood IGF1 and leptin levels in infants of mothers with a history of gastric bypass have also been reported [34].

Although there is conflicting evidence regarding the effect of bariatric surgery on delivery type, it appears that women who become pregnant after bariatric surgery have a higher risk of cesarean delivery compared to healthy controls [35,36,37] and lower risk compared to controls with obesity [38, 39], as was the case for our study. Higher maternal age and higher prevalence of previous cesarean delivery could explain the higher risk of cesarean delivery reported in some studies [9, 40, 41]. Caregiver bias has also been proposed as a contributing factor [35, 42]. On the other hand, lower BMI and lower incidence of gestational diabetes and LGA neonates after bariatric surgery could contribute to a lower risk of cesarean delivery[40, 43]. A lower risk of labor dystocia in postbariatric pregnancies, possibly in relation to smaller neonates, has also been reported [9, 41].

Evidence regarding the effect of bariatric surgery on gestational weight gain is conflicting. While most studies suggest that BG leads to lower gestational weight gain [9, 44,45,46], others point to similar or even higher gestational weight gain [10, 47]. These disparities could be due, in part, to differences in the characteristics of control groups in each study.

In women without history of bariatric surgery, excessive gestational weight gain is known to increase the risk of LGA neonates, cesarean delivery, gestational diabetes, and hypertensive diseases of pregnancy [48]. In our study, pregnancy after bariatric surgery was associated with higher gestational weight gain, compared to controls with BMI ≥ 35 kg/m2, but there was no increase in the above-mentioned outcomes. In addition, insufficient gestational weight gain is known to increase the risk of preterm birth and SGA neonates [48], and our study confirmed that in postbariatric pregnancies gestational weight gain < 10.0kg leads to higher risk of preterm birth. Similarly, in a bicentric retrospective study conducted by Grandfils et al., postbariatric pregnancies with inadequate gestational weight gain were at higher risk of preterm delivery [49]. Furthermore, Stentebjerg et al. found that women who conceive within 18 months after surgery have less gestational weight gain, which is consistent our results regarding timing of pregnancy and gestational weight gain.

In the authors’ opinion, some explanations for the higher gestational weight gain observed in the bariatric surgery group overall could be that the dietary plan prescribed to pregnant women in our center is of similar nutritional value across different prepregancy BMIs, and the fact that women with a higher prepregancy BMI are actively encouraged to avoid excessive gestational weight gain [50].

Importantly, there is still debate regarding the ideal gestational weight gain in postbariatric pregnancies, and whether current guidelines can be applied to this specific population[14, 49].

Due to conflicting evidence in the literature, in our center, women who undergo bariatric surgery are encouraged to wait a minimum of 12 months before becoming pregnant, and subsequently to maintain an adequate gestational weight gain, in accordance to the 2009 Institute of Medicine guidelines [50].

The retrospective and observational nature of the study limits its interpretation. Also, the sample size might have been insufficient to detect differences between groups in some outcomes. This could explain some disparities in our results and the literature.

In our study, adjustment was made for BMI before surgery in the bariatric surgery group, and BMI before pregnancy in the control group. Only women with a BMI of ≥ 35 kg/m2 were included in the control group, as they would be considered bariatric surgery candidates. This was done, as in other studies, to assess the effect of this procedure on women with a BMI ≥ 35 kg/m2, who presumably would have carried out future pregnancies with a similar BMI if not for the surgery [10, 41, 47]. This assumption could, however, limit the interpretation of the results, as some pregnancies occur several years after bariatric surgery, and weight changes could happen during this time interval.

Finally, another limitation is the fact that micronutrient status during pregnancy was not evaluated. Bariatric surgery is known to increase the risk of micronutrient deficiencies [34, 51], and nutritional status of the mother seems to play a role in fetal development [33]. There is still limited evidence, however, on the impact of micronutrient deficiencies in postbariatric pregnancies on perinatal outcomes.