Back in our ancestral times when our tribal forefathers roamed the savannas, food was scarce. Those who did behavior that increased caloric intake had a distinct survival advantage over those who were less enthusiastic about increasing caloric intake. This is why we all inherited genes that make us gravitate towards sweet foods and fatty foods. However, those very genes that gave a survival advantage back then in an environment of scarcity nowadays give us a survival and reproductive DISADVANTAGE. Sure, if we were all sitting around in semi-starvation mode, those of us who are more driven to seek something sweet to munch on would be healthier than those of us who continue to waste away. But our present day environment is not like that. Right this moment, I bet that pretty much all of you could get your hands on an instant 1000 calories within 30 minutes. I could probably do so within 30 seconds as long as I’m willing to brave raiding my nurses’s fridge. In this state of plenty, those of us who have the will power to RESIST sweets and fats end up healthier and more fertile.
In general the optimal BMI for fertility is around 21, according to a recently published book on Fertility and Diet. To get your BMI, you can use this online calculator. As women who suffer from PCOS often learn, obesity is detrimental to fertility by affecting egg quality and therefore embryo quality. If we are careful not to think in absolutes, we can then accept that many thin women get pregnant and many obese women get pregnant, many thin women are infertile and many obese women are infertile. But, IN GENERAL, your chances of conceiving and delivering a healthy baby get better as you get closer to a BMI or 20-24. In general, obese women have a higher chance of miscarriage and a higher chance of obstetric problems during their pregnancies.
Besides affecting egg quality, does obesity affect fertility in other ways? For example, does it affect the lining of the endometrium and its ability to sustain implantation? If so, is it the egg quality that is hurt by excess body fat, or is it the lining that is hurt by it? Well, in the past, we could not have answered this question, because all obese women would make "obese-woman eggs" and carry them in their "obese-woman lining". There was no way to separate the two. Now, however, with the advent of egg donation, we can study the effect of obesity on the lining separate from the effects of obesity on eggs. A recent journal article reports on findings that do not paint a ROSY picture for those who carry excess body fat.
In a recent paper out of Spain, researchers looked at women who were undergoing IVF with donor eggs. The beauty of this study is that the researchers were able to separate the effect of embryo quality from the effect of the body that was carrying the embryos. Because all the patients had an equal chance of getting good embryos from donors, then the differences in success rate, if any, could be better attributed to factors about their own bodies independent of any egg factors.
This ambitious study included data for 2656 cycles from the year 2001 to 2005. The overall pregnancy rate for these egg donor IVF cycles was 59% with a miscarriage rate of 16%. But there was a marked difference between women of different BMI groups. The study divided women into 4 categories of BMI: THIN = under 20. NORMAL 20 to 24.9. OVERWEIGHT 25 to 29.9. OBESE = greater than 30.
| Thin | Normal | Overweight | Obese | |
| Pregnancy Rate | 60.3% | 60.1% | 56.6% | 49.2% |
| Miscarriage Rate | 14.8% | 15.9% | 19.7% | 18.3% |
| Ongoing Pregnancy Rate | 46.7% | 45.2% | 38.9% | 36.1% |
As you can see, those in the obese group did the worst overall with regards to the chance of taking home a live baby.
The researchers concluded three things.
- THE OVARY IS NOT THE ONLY FACTOR RESPONSIBLE for the poor reproductive outcome in obese women.
- UNDERWEIGHT WOMEN DO NOT SUFFER FERTILITY IMPAIRMENT for uterine reasons. While it’s true that underweight women do have more fertility problems than normal women, it seems that this is related to ovulation problems, which in this study, did not come into play.
- BEING OVERWEIGHT implies a more negative reproductive outcome. Therefore, women should be counseled about weight control.
Personally, my patients know that I am pretty aggressive about encouraging women to control their weight and get healthier when pursuing having a child. However, I do promise to do it in a non-nagging way 
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