How does obesity affect my fertility?

By Heather

If you're trying for a baby then you may have to wait longer if you are obese researchers say. (1) This is mainly a hormonal effect and can be helped by weight loss. 

For women, for every unit increase in BMI the chance of conceiving naturally decreases by about 5%; so for example, if your BMI is 31 you have 5% less chance of natural conception than if your BMI is 30. In one study looking at young women aged 18 years old with a BMI of 32, the risk of infertility was almost trebled compared to their peers with a normal BMI. (2) 

The medical condition PCOS (polycystic ovarian syndrome) is associated with similar hormonal profiles to obesity. This condition is characterised by irregular menstrual cycles and below normal fertility levels. Not all PCOS sufferers are obese. However in those who are the hormonal irregularities are magnified.

In men, a similar disruption of the normal hormonal profile also occurs. This effects sperm quality and reduces fertlity directly and with reduced testosterone affecting fertility indirectly through decreased libido and erectile dysfunction. The only proven treatment to increase the likelihood of pregnancy is weight loss. (3)

Once you fall pregnant, the risk of miscarriage in early pregnancy (less than 12 weeks) is increased if you are overwight or obese. This is true whether you fall pregnant naturally or following assisted reproductive techniques such as IVF. (4) The reasons for this are currently being intensely researched and the verdict is still out. In some countries, your access to publicly funded IVF treatment if you are an obese woman may be restricted because of treatment failure rates. However although the UK guidelines suggest a BMI of less than 29 is preferred, practice varies locally. (5)

What can be done? As has been mentioned already, weight loss is the only known treatment to work; there are no medications or interventions which have any proven effect. Even modest weight loss - in the region of 5 - 10% can help your fertility. In one study, 4 out of 5 obese women with infertility and hormonal irregularities regained normal menstrual cycles with 1 in 3 of them becoming pregnant. (6)

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References
1. Wise LA et al An internet-based prospective study of body size and time-to-pregnancy. Hum Reprod 2010;25:253-64.      
2. Rich-Edwards JW et al Adolescent body mass index and infertility caused by ovulatory disorder. Am J Obstet Gynecol 1994;171:171-7
3. Hammoud AO et al Obesity and male infertility: a practical approach. Semin Reprod Med. 2012  Dec; 30(6): 486-95
4. Wang JX et al Obesity increases the risk of spontaneous abortion during infertility treatment. Obes Res 2002;10:551-4.
5. National Institute for Clinical Excellence. Fertility guideline: Assessment and treatment for people with fertility problems. http://www.nice.org.uk/CG011
6. Hollmann M, et al Effects of weight loss on the hormonal profile in obese, infertile women. Hum Reprod 1996;11:1884-91