Many women are aware that very low body weight and low percentages of body fat can compromise ovulation and chances for successful conception. What many don't realize is that excess body fat can also affect one's chances.
A review of the literature shows that the majority of studies published report decreased chances of conceiving with in vitro fertilization (IVF) if a woman in significantly overweight. IVF data is useful to study this issue because all the women undergo similar treatments and because follow-up data on pregnancies is usually readily accessible to researchers. It may also be true that excess body weight is a negative factor in spontaneous conception and non-IVF treatment as well.
How much of a factor is weight in decreasing conception? One study from the Netherlands reported a higher cycle cancellation rate due to poor response to stimulation and lower fertilization rates1 than normal weight women. Another study from Norway reported higher requirements for stimulation medications and a higher miscarriage rate in the first 6 weeks of pregnancy2. One of the largest studies was from Cornell and reported on 960 women undergoing IVF. Although they did not find a statistically significant decrease in clinical pregnancy rates, rates of cycle cancellation were higher and gonadotropin dose requirements were greater in the obese patients3. Another large study from Iowa (1,293 patients) looked at outcomes in women who were obese and morbidly obese. Again, this study found that clinical pregnancy rates per egg retrieval were similar to normal-weight women but cancellation rates and gonadotropin dose requirements were much higher in the obese women. Furthermore, rates of pregnancy complications such as preeclampsia, gestational diabetes and cesarean section were higher in the obese women4.
How much weight is significant for this effect? Most studies calculate weight as Body Mass Index, or BMI. This calculation takes in weight vs. height. To calculate your BMI, many websites such as the one at the Centers for Disease Control ( www.cdc.gov/nccdphp/dnpa/bmi/index.htm) can provide a calculator. There is also a chart at the federal government's website www.consumer.gov/weightloss/bmi.htm. You just need to know your height in feet and inches and weight in pounds. A normal BMI is between 18.5 and 24 and overweight is a BMI of 25 to 30. A BMI of 30 or more is considered obese and 40 or more is considered morbidly obese.
In general, it appears that excessive body weight can negatively impact a woman's chances for conception and for a healthy, uncomplicated pregnancy and birth. It makes sense that being a normal body weight and in good shape is a good idea and should be a goal for aiding successful conception.
Carolyn Givens, MD
References 1. Gynecol Obstet Invest. 2005;59(4):220-4. Epub 2005 Mar 7. Obesity and Clomiphene Challenge Test as predictors of outcome of in vitro fertilization and intracytoplasmic sperm injection.van Swieten EC, van der Leeuw-Harmsen L, Badings EA, van der Linden PJ.
2. Hum Reprod. 2004 Nov;19(11):2523-8. Epub 2004 Aug 19. Impact of overweight and underweight on assisted reproduction treatment. Fedorcsak P, Dale PO, Storeng R, Ertzeid G, Bjercke S, Oldereid N, Omland AK, Abyholm T, Tanbo T.
3. J Reprod Med. 2004 Dec;49(12):973-7 Obesity and in vitro fertilization: negative influences on outcome. Spandorfer SD, Kump L, Goldschlag D, Brodkin T, Davis OK, Rosenwaks Z.
4. Obstet Gynecol. 2006 Jul;108(1):61-9. Obstetric outcomes after in vitro fertilization in obese and morbidly obese women. Dokras A, Baredziak L, Blaine J, Syrop C, VanVoorhis BJ, Sparks A.