Weight seems to have a negative influence on fertility. There certainly are women who are overweight, achieve a pregnancy easily, and have children. But if a woman is having problems conceiving and she is overweight, she may want to consider that her weight is part of the problem.
Does increased weight reduce a woman’s chance of conceiving?
What does it mean to be overweight? The World Health Organization (WHO) defines overweight as a Body Mass Index (BMI) as > 25 kg/m2 and obese as a BMI >30 kg/m2. Presently, in the US it is estimated that >25% of the women in the reproductive age range are obese. Obesity is associated with a problem where women have either no periods (menses) or very irregular menses. Most of these women have polycystic ovarian syndrome (PCOS). For women with PCOS who are obese, over 70-90% have weight-related insulin resistance, which just means that because of their weight their insulin is inefficient and they release more insulin than they should. However, women who have fairly regular menses but are overweight also have a reduced chance of conceiving due to their weight. A study by van der Steeg (Hum,. Reprod. 2008) established a reference range for women of a BMI between 21 And 29 kg/m2. Compared to this reference range, women with a BMI >35 had a 26% less chance of spontaneously conceiving. For women whose BMI was >40 kg/m2, the reduction in the chance of having a spontaneous pregnancy was 43%. Looked at in a different way, even if a woman’s BMI was >40 kg/m2, she had a 57% chance of conceiving. However, when woman conceive spontaneously, they do so quickly- usually within three to six months. So, if a woman has been trying for over a year to conceive, and she has not been successful AND her BMI is >35 or 40 kg/m2, it just might be the weight that is the problem. In practical terms, if a woman has been trying to conceive for over year and there is nothing else wrong, but she is 5’3” and weighs more than 165 lbs., her weight might be preventing her from conceiving. For a woman 5’5”, the critical (maximum) weight is about 175 lbs., and for women 5’7”, the critical weight is 185 lbs.
Notice that one of the caveats is that all other things are normal. Achieving a successful pregnancy requires a number of things to be normal. Therefore, before deciding that weight is the problems, a woman needs to eliminate male factor as part of the problem. Also, she needs to have normal anatomy (structure) to conceive, so some form of an evaluation of the structure of the uterus, ovaries, and fallopian tubes is required. Sometimes there will be more than one factor reducing a woman’s chance of conception, so attention to all factors will increase the person’s chance for conception.
Why is weight control such a problem?
A major factor influencing weight is genetic. Some studies show that up to 60-70% of obesity is hereditary. For hereditary factors, there has to have been an evolutionary reason for the problem. A commonsense explanation would be that the early ancestors of humans lived in an environment of calorie scarcity, so there was no need to regulate…no one was obese. However, comforting that explanation is, it seems to be wrong. Currently there are three major theories about the evolutionary genetic control of weight. The most comprehensive sees the genetic evolution of weight as a balance between calorie seeking and weight reduction. The argument for weight reduction suggests that as weight increases, animals become slower and thus more susceptible to predators. While that seems fantastical, there is evidence that early hominids were a favorite food for saber-tooth tigers, so looking at the problem from an early Sapiens perspective, being overweight might just be a fatal flaw. On the other hand, increased weight has been associated with increased ability to fight disease. Again, not such a big deal today, but as an early Sapiens, staying healthy was no minor feat. The hypothesis says that the two situations are controlled by different mechanisms. The drive to gain weight involves the hormone Leptin, and suggests that there is a set-point weight below which leptin will drive calorie seeking. The control of the upper limit is not leptin dependent, but the actual control is not currently defined. The sum of this theory is that weight control is much like a wide highway where weight can vary between extremes, but that overall, there is a constant genetically determined direction.
The significance of these theories is that for the short term, people can control their weight. However, over the long- haul, weight is genetically determined and very difficult to alter.