Obesity is an increasing epidemic disease in the industrialized world due to changes in lifestyle, reduced exercise and dietary habits. Other factors such as endocrine, hormonal or physiological disorders may lead to this disease. Nowadays, the World Health Organisation has described obesity as a body mass index (BMI) over 30 Kg/m2 and it is associated with cardiovascular disease, diabetes, osteoarthritis and malignancies such as endometrial or colon cancer.

Also, this disease is currently listed as one of the causes of infertility in women. But, why obese patients tend to find more difficulties to get pregnant, both for natural and artificial conception? Numerous studies have described a delay in natural conception in obese women, mainly due to an increased risk of anovulation. However, even in obese women with regular cycles, it has been observed that the probability of pregnancy accumulated over a year is reduced by 5% for each unit of BMI that exceeds 30 Kg/m2. These findings could suggest the absence of oocytes in spite of the existence of regular menstruations, the release of oocytes of low quality with a reduced potential of fertilization or the alteration of the endometrium.


Scientific research has demonstrated that hormones found in the gut and fatty tissue have an important role in the regulation of reproductive function. In cases of high caloric intake, which increases the fatty tissue, the level of those hormones is higher, interfering negatively in the hormone cascade that controls reproductive functions. Obese women’s hormones are out of balance and cysts are formed in the ovaries, leading to polycystic ovary syndrome. Consequently, women suffer from menstrual irregularity, hirsutism (excessive hair growth) and infertility. They also present higher probability of developing further affections such as diabetes or heart disease than average women.

Moreover, when a woman is stimulated in order to obtain oocytes, the treatment to be applied is directly related to her weight. In women suffering from morbid obesity (BMI over 40 Kg/m2), the dose of hormones to be administered needs to be significantly increased and, consequently, the risk of suffering from possible adverse effects such as ovarian hyperstimulation syndrome is also higher. Thus, this risk is especially important in obese women with polycystic ovaries, where the chances of achieving pregnancy through assisted reproductive techniques are significantly reduced.

Consequently, female obesity is associated with fewer follicles, defective or reduced oocytes and embryos and lower live birth rates. Oocytes from overweight women present specific defects on metabolic pathways, which implies potentially impaired development of children, compared to those from women of BMI within the healthy range. These oocytes from overweight and obese women are smaller and less likely to reach the blastocyst (stage of development at which embryo undergoes implantation in the endometrium). However, those that manage to do so normally show accelerated preimplantation development and the subsequent blastocysts contain fewer cells, notably in the trophectoderm (the structures that will give rise to extra-embryonic tissues and placenta).


Obesity during pregnancy has short- and long-term adverse consequences for both mother and child. In early gestation, maternal obesity significantly increases the risk of complications, such as hypertensive disorders, gestational diabetes, preterm delivery, respiratory diseases or thromboembolic complications. It might cause spontaneous pregnancy loss and congenital anomalies. Metabolically, obese women have increased insulin resistance in early pregnancy, which is clinically manifested in late gestation as glucose intolerance and foetal overgrowth.

At the end of pregnancy, the risk of cesarean delivery and complications at labour is greater in obese women.  In addition, the altered metabolism of the embryo can be the cause of an overweight child at birth.


In men, obesity also has negative repercussions on their fertility. It has been suggested a relationship between the increase in BMI and the decrease of the concentration and mobility of their spermatozoa. This could be explained by the increase in fat tissue, which causes an imbalance of testosterone, one of the main hormones that play important roles in the maturation of spermatozoa. In addition, obesity can also lead to cardiovascular complicationsdiabetes or joint problems that are related to a decrease of semen volume and altered sperm DNA. These negative DNA changes may be related to lower pregnancy rates and increased risk of miscarriage, and can also lead to some serious birth defects.


Since there is evidence enough to affirm hat overweight influences fertility and fertility treatments, it is important to monitor health to avoid problems in general, and in particular those which may affect conception. If you have trouble conceiving it is always good to seek professional help and consult a specialist.


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