Being fat reduces a woman's chances of getting pregnant and having a successful pregnancy by more than a quarter, and the fatter she the less likely to conceive. Being merely overweight reduced chances by about a seventh, and being really fat reduced it by more than a third. The study involved 50,000 women using assisted reproductive technology, but they say it applies to all women.
The study's author said, "The results are not surprising; obesity is a state of inflammation and is not a good environment for conception or fetal development." I guess that means that fat people have a lot of inflamed tissues, and that causes the problems with conception an carrying.