OK, here we go - for CSWS.
Basic and very quick trek through some literature suggests that BMI >30 is related to anovulation and some increased risk of early miscarriage, possibly through hormonal changes (something to do with leptin?). So if obese and having trouble TTC, much of the research suggests try and loose weight. Couldn't find any stats for increased infertility risk as such, but there's a fair bit out there on the mechanics of why this might be so, and on weight / IVF / ISIC problems.
A Dutch study suggests that even in women with normal cycles (and partners with normal sperm counts) there is a relationship between BMI >30 and taking longer to conceive:
(1)Study of 3000+ couples in the Netherlands showed that, compared to women with a BMI between 21 and 29 (normal weight or overweight), women with a BMI of 35 had a 26% lower chance of pregnancy, women with a BMI of 40 had a 43% lower chance. For every BMI unit over 29, there is a drop of 4% in the chance of conception.
BUT an American study suggests this is only a problem if women ALSO smoke:
(2) American study found a strong association of BMI over 30 and increased time to achieve pregnancy ONLY in smokers (who were more than 10* as likely as non obese women to take more than 9 months to get pg).
So not cut and dried. The research suggests a risk of finding it harder to get pg if BMI>30, but actual figures are few and far between. Also read something that essentially said age is more of a factor than weight.
More clear cut is the relationship with problems in pregnancy, but risks are usually presented as odds ratios, not absolute risks, so I don't want to start brandishing numbers around without the time to go through the lit properly.