After reading LB's post on her success with becoming pregnant after losing weight, I started to research weight and fertility. I read today that both being overweight and underweight can affect your fertility by upsetting the hormonal balance in your body. For women, being overweight interferes with normal egg production, and for men, with normal sperm production.
Here is the article on the effects of body weight on fertility:
http://www.pregnancymd.org/weight-and-infertility.htm