There is a definite association between a woman's weight and her capability of becoming pregnant. Being overweight oftentimes affects the menstrual cycle, causing irregular cycles or straight out amenorrhea (lack of periods).
The underlying mechanism responsible for this is lack of normal ovulation. Women who are overweight or obese are significantly more prone to ovulatory problems as compared to women of normal weight (see the explanation below). In other words, overweight women may either have a poor quality ovulation, or may not ovulate altogether. Under these circumstances, since no egg is produced by the ovaries, conception is impossible.
Why does obesity affect ovulation? Well, here's the medical explanation: Fat cells naturally produce estrone, which is a hormone of estrogen type. This occurs during the normal metabolism of fat cells, due to the conversion of a precursor hormone called DHEA into estrone. The more fat cells one has in the body (i.e., the greater one's weight), the more estrone will be generated by these cells and released into the bloodstream. This in turn suppresses the hypothalamus and pituitary gland (which are responsible for regulating the ovarian function), and leads to ovulation problems including anovulatory cycles (i.e., absence of ovulation).
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This link between excessive weight and fertility is scientifically proven and has been amply documented. Lately, however, studies have proven that fertility may be affected even in overweight women with normal ovulatory function. The mechanism responsible for this is not clear, but is quite likely linked to a poor quality of ovulation, probably due to the altered balance of hormones in the organism (i.e., too much estrone versus estradiol). Estrone, the hormone produced by fat cells, is a weaker estrogen hormone than estradiol (the hormone produced by the ovaries). The altered ratio of these hormones in overweight women may lead to ovulation of poor quality, i.e. the release of an egg which is not ready to be fertilized.
Even if such an egg is fertilized, the implantation of the fertilized egg in the uterine wall may be impaired. In other words, due to the altered hormonal balance in the body, the uterus is not ready to accept the fertilized egg. In such cases, the product of conception is lost in the very early stages (days after fertilization occurred), and the woman is never aware that fertilization even took place.
To take it one step further, overweight women who do get pregnant are at greater risk for miscarriage and other pregnancy complications, including pregnancy-induced diabetes and hypertension.
The fertility treatments currently available rely on stimulation of ovarian hormone production in the body, which may unfortunately result in even more weight gain. Therefore, an overweight woman who successfully undergoes fertility treatments is quite likely setting herself up for a higher risk pregnancy.
In conclusion: Excessive weight is a problem that should be addressed prior to getting pregnant. A judicious weight loss program, including vitamin and mineral supplements, as well as Calcium, Iron, and Omega3 supplements, may restore the normal ovulatory function and avert the need for fertility treatments altogether.
Anecdotally, the vast majority of women affected by infertility which I encountered in my practice were also overweight or obese. Most were not aware or chose to ignore their weight problem as a cause for their failure to conceive.
Again, when seeking help from a fertility clinic, it is quite likely for the weight problem to be entirely overlooked, and for the patient to be started right away on heavy-duty hormonal treatments. So, one has to make an individual choice to address excessive weight prior to seeking such treatments.
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