In SOME cases being overweight can stop ovulation. And losing the weight can bring ovulation back. Women with PCOS are more likely to be overweight... so thats another reason for the link between weight and fertility issues (plenty of skinny girls out there with PCOS though).
I'm in the morbidly obese category and I did have trouble concieving. However my GYN, and my specialist both did the shock/double take at how healthy my BP and bloodwork showed I am. My specialist did say that it never hurts to lose weight, but felt that it wasn't causing my issues.
I did happen to lose about 20lbs in the past 7 months or so, but I know the reason I got my BFP was that I finally had high enough progesterone thanks to the right dose of clomid.
The only downside, is that I am going to be watched like a hawk for weight gain, lol! So in that sense, losing some weight before pregnancy means you don't have to be AS strict while pregnant or at least don't get hounded by the doctors for an extra 5lb gain. They want me to keep my weight gain as minimal as possible.
Which is a little hard when the morning sickness makes almost everything unappetizing besides carbs
Its all about the portion control though and I'm doing ok so far.
I'm only concerned about gaining too much because my body LOVES to put on weight and because I know the bigger risks are in gaining TOO much while pregnant. So we'll just have to do the best we can. I'm not particularly worried about it, just have that much more pressure to eat healthy which is a good thing for many reasons.