I have a friend who became pregnant twice while being mostly vegan (with a little egg) and having very low body weight and fat; the first time was accidental, and the second time happened the very day she started trying, so it is clear that she had no fertility problems at all.
I am a vegan with a moderate amount of body fat, and I spent ages 33 through 35 trying to conceive. I was only successful after taking Clomid, and I have no idea if my diet was a factor. I did decide to almost entirely eliminate soy towards the end, after reading this article concerning genistein:
http://news.bbc.co.uk/2/hi/health/4115506.stm
I'm back to having soy once per week now, and it's mostly tofu. Unfortunately, my moderate consumption of soy before reading this article was mostly tempeh, because I thought it was better for you. It might be, in some ways, but it contains more available (accessible by the body) genistein than tofu.
Otherwise, I don't know much about veganism and fertility, but I do know how to gain weight as a vegan. Just eat like me! Pancakes with syrup and Earth Balance, all-you-can eat dosa nights (at a South Indian restaurant), cookies, French fries, huge portions of everything, etc. (I do eat healthy food, too, but I am giving you some fattening examples.) At around 22 weeks of pregnancy, I had already gained 20 pounds (and I was of average weight before - not skinny), and my midwife was shocked when I told her what I'd eaten that day. I believe it was a large portion of dinner leftovers (white bean vegan quiche), something involving noodles and veggies for lunch, and a huge cookie for dessert. She told me that the carbs were making me gain weight, so I started incorporating things like seitan over greens (without bread or rice) as dinner occasionally.
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