Obesity and fertility

Obesity and fertility

The extent of the issue

Obesity is a public health issue both in the general population and in women of childbearing age. The figures are a proof of this and, far from improving, the problem has been getting progressively worse over the last few years.

Maternal obesity in the United States of America was calculated to be 7% in 1980 and had risen up to 24% by 1999. If we take into account both obesity and excess weight, the available data indicates that the figures were 37.1% in 1999 and 40.5% in 2003.

The NHANES (National Health and Nutrition Examination Survey) reveals obesity figures (BMI or body mass index ? 30 kg/m2) in women of reproductive age (20 to 39 years of age) of 31.9%.

In other words, one in every three women of a fertile age is obese.

The impact of obesity on fertility

Most studies show a link between an increase in BMI and subfertility.

Fertility issues in obese women are, in most cases, linked to ovulatory dysfunction and, in some cases, to polycystic ovary syndrome. Furthermore, it should be highlighted that in women with normal ovulatory cycles, obesity itself is linked to a lower spontaneous pregnancy rate and an increase in the time taken to get pregnant. This is an important factor in this day and age when maternity is being delayed more and more due to social constraints. Some studies show that the levels of anti-Müllerian hormone (the hormone that indicates ovarian reserve) are 34% lower in women between 18 and 35 years of age who are obese and have normal ovulatory cycles in comparison with women who are not overweight.

What this tells is that obese women will have more issues getting pregnant naturally, will take longer to do so and also have less real time in which to do so, even if they have apparently normal cycles, because they appear to have a lower ovarian reserve.

Ovulation induction in obese women

The data is contradictory. Some studies conclude that results in courses of treatment are worse in obese women (insufficient follicle development and a lower oocyte count) whilst others indicate similar results in obese women and women who are not obese. The latter only highlight the need to use larger doses of ovulation induction drugs.

Assisted reproduction treatment (ART) in obese women

The data suggests that, as woman’s BMI increases, there is an increased failure rate following ART (IVF: In vitro fertilisationICSI: Intracytoplasmic sperm injection). This has been linked to a decrease in the quality of the oocytes obtained, worse ovarian response, worse endometrial quality or a combination of these factors.

A meta-analysis analysing 48,000 cycles of IVF/ICSI confirms a lower clinical pregnancy and live newborn infant rate and a higher spontaneous pregnancy loss rate in women with a BMI > 25 kg/m2, in comparison with women who have a normal BMI. An increased number of other types of complications, such as ectopic pregnancies, ovarian hyperstimulation syndrome and so on, has not been demonstrated.

The impact of obesity on implantation and pregnancy loss rates in obese women who are receivers of donated oocytes is being studied. Preliminary data has provided contradictory results. Therefore, the question of if the link between obesity and subfertility is due to ovarian dysfunction, oocyte quality or endometrial receptivity is yet to be resolved. Amongst these factors, the latter would appear to be important. Some studies show that weight loss amongst obese women who have had recurrent pregnancy losses does improve their chances of a full term pregnancy once they have lost the weight.


Several observational studies have found that weight loss in obese women with fertility issues improves ovulation frequency and the chances of getting pregnant. The suggestion would seem to be that all obese women who wish to get pregnant should consult a team of experts in the field so that they can assess their case taking into account all the factors that are in play (the mother’s age, ovarian reserve, the extent of the obesity issue and possible consequences). By doing so, they can design a strategy with realistic, personalised and appropriate objectives.

Dr Pino Navarroendocrinologist. Director of the Endocrinology and Nutrition Department at Instituto Bernabeu

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