There are two sources of estrogen in the body: the ovary (directly) and the adrenal gland (indirectly). The ovary makes estrogen in phases--up and down--in what are part of the variations within the normal menstrual cycle. The adrenal glands, among other things (like adrenaline), make something called "androstenedione." All of these hormones are related to cholesterol, actually--even testosterone and progesterone, so it's common that hormones often are converted back and forth. In the case of the adrenal androstenedione, fat cells can convert it into an estrogen called estrone.
Besides causing increased risk of heart disease, hypertension, and diabetes, obesity can also cause biochemistry to gang up on your chances for decent ovulation. If you're significantly overweight, while your ovaries are trying to behave and cycle normally, the steady input of estrone from this other pathway will interfere. A steady supply of estrogen from the peripheral conversion of androstenedione to estrogen will blunt the peaks and valleys of the ovary's function, which ultimately interferes with ovulation and can cause infertility.
Also, all of this estrogen can even lead to pre-cancerous changes in the womb (uterus). These changes are usually reversible, but nevertheless any woman who is overweight and experiencing menstrual irregularities should be evaluated.
Knowing The Affect Of Weight
How can you tell whether you're a woman whose ovulations are affected by being overweight? If you're having regular periods, then you can assume you're probably having regular ovulations. But if the periods are sporadic, then probably so are your ovulations. If your periods are continuous or absent, then any expectation of ovulation will be unreliable.
Thyroid problems can cause both irregular periods and obesity, so it isn't unreasonable to begin your infertility work-up with blood work to check for this. Thyroid problems are easy to diagnose and usually easy to fix, but if this abnormality isn't ruled out and you have this problem, then hormonal manipulation probably won't help.
Being overweight affects everything. You can take lists of diseases from every specialty and find some obesity-related ones in each. Hypertension, coronary artery disease, pulmonary problems, diabetes, skin health...the benefits of losing weight are significant. One of them is being able to ovulate regularly and increasing your chance of fertility.