Women who have had bariatric surgery appear to be at higher risk of preterm and small for gestational age (SGA) babies, a large study from the Karolinska Institute in Sweden has found.

The Swedish birth registry captures data on 98% of all births nationwide. Between 1992 and 2009 more than 1.7 million births were identified, after relevant exclusions, of which 2,562 were to women who had undergone bariatric surgery for obesity, and had a singleton pregnancy.

Women who had had bariatric surgery were on average older, more parous, more likely to be obese and to be smokers.

Five controls were selected for each woman with a history of bariatric surgery closely matched for age, parity, BMI and smoking status in early pregnancy, education and year of delivery.

A secondary control cohort including women eligible for bariatric surgery, BMI ≥ 35 or ≥ 40 kg/m², but who had not had the procedure were also matched for the same factors except BMI.

Preterm and SGA births occurred more often in women with previous bariatric surgery, see table 1. However, the incidence of large for gestational age births was significantly less. Stillbirth and neonatal death rates were equivalent. The increased risk of preterm and SGA births in the bariatric surgery group women was confined to those with a BMI < 35 kg/m².

Two further multivariate analyses were conducted comparing all bariatric surgery group women with the control group women with BMIs ≥ 35 and ≥ 40 kg/m². This allowed comparison of control group women who would be eligible for bariatric surgery with women who had actually had the procedure performed. Similar but attenuated trends to higher rates of preterm birth and SGA were seen in the bariatric surgery group women compared with controls with BMI ≥ 35. However, compared with controls with BMI ≥ 40 only low birthweight remained significantly higher in bariatric surgery women.

Neither the type of bariatric surgical technique (gastric band, bypass or gastroplasty) nor the year in which it was performed had an effect on birth outcome data.

This large well conducted study confirms the findings of previous studies showing that women who have had bariatric surgery are at higher risk of preterm delivery and a SGA baby. The more successful the surgery, in terms of BMI at the start of pregnancy, the greater the risk appeared to be. The causes are not clear. The authors speculate on altered micronutrient intake after surgery, but no changes to clinical practice, other than including their conclusions in pre-surgery counselling, were made.

The protective effect of bariatric surgery against having a large for dates baby suggests that the known improvement bariatric surgery has on insulin sensitivity may be effective during pregnancy too.

Dr Chris Barclay

• Roos N, Neovius M, Cnattingius S et al. Perinatal outcomes after bariatric surgery: nationwide population based matched control study. BMJ 2013;347: f6460