A report in the QJM (the monthly journal of the Association of Physicians), which examines the effects of gestation on weight loss outcomes, concludes that pregnancy after bariatric surgery is safe and does not adversely affect weight loss outcomes: http://www.bariatricnews.net/?q=opblog.
I suspected that the published paper which made this conclusion, may have had a predominance of gastric band operations, however on closer inspection, more than 80% of the operations performed were gastric bypass procedures. Twenty one women went on to have 28 pregnancies following weight loss surgery, of which 85% were live births. The authors rightly concluded that pregnancy following bariatric surgery is safe, including gastric bypass surgery.
