Obesity is a known, and common, risk factor for infertility. Most studies have shown that women with obesity have markedly reduced success with fertility treatments (also called assisted reproductive technology, with examples include artificial insemination and in vitro fertilization).  The next natural question, then, is: does weight loss before assisted reproduction in these women improve pregnancy rates?
This question was addressed in a review article recently published in Obesity Reviews. Overall, they found that the quality of the available data is weak, with few patients studied and few studies done. They did find that the majority of the studies, which looked at a variety of means to achieve weight loss before assisted fertility techniques were undertaken, showed improved pregnancy and/or live birth rates.
It is sometimes asked whether a more drastic means of weight loss, such as the very low calorie diet (VLCD) or bariatric surgery is appropriate to improve fertility. With regards to the VLCD, they found conflicting data the two studies that looked at it, with one study showing improved pregnancy rates, and the other showing particularly poor fertility outcomes. This may be related to the fact that the successful study had a follow up (less stringent) diet after the VLCD was complete, whereas the other study gave no guidance after the very low calorie phase was complete. (my editorial comment – VLCDs are not recommended in any case – read more here).
They located two studies of women having bariatric surgery before assisted reproduction (note: only 6 patients total), with excellent results for improving pregnancy rates.  (Note that after bariatric surgery, it is important to wait 1-2 years before conceiving for safety reasons.)
Perhaps the most important findings that came from this review is that of six studies that evaluated whether anyone became spontaneously pregnant, five of the studies reported that pregnancies did occur without needing any reproductive treatments, in women who had previously been unable to conceive at a higher body weight.

So, based on current knowledge, it seems that weight loss is of benefit in women who struggle with excess body weight, both for improving spontaneous conception, and also for conception by assisted reproduction.
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