Can obesity cause infertility?

First of all, what is infertility?

Infertility is a disease of the female and male reproductive system defined by the failure to achieve a pregnancy after 12 months or more of regular unprotected sexual intercourse. Unfortunately, infertility affects up to 15% of couples trying to get pregnant and can have multiple causes, including genetics, general health, fitness, diseases and diet. Moreover, many studies suggest that obesity can cause infertility.(1)

Five causes of female infertility 

As mentioned earlier, female infertility refers to women trying to get pregnant with frequent, unprotected sex for at least a year with no success. A variety of factors that affect or interfere with the following biological processes include:

• Ovulation is when the mature egg is released from the ovary. The most common overall cause of female infertility is the failure to ovulate, which occurs in 40% of women with infertility issues.(2)
• Implantation failure refers to the failure of a fertilised egg to implant in the uterine wall to begin pregnancy. While the specific cause of implantation failure is unknown, possibilities include genetic defects in the embryo. 
• Structural problems occur when the fallopian tubes are blocked, eggs are not able to move from the ovaries to the uterus, and sperm is not able to reach the egg for fertilisation.
• Polyps are noncancerous growths on the inside surface of the uterus. Polyps can interfere with the function of the uterus and make it difficult for a woman to remain pregnant after conception. Surgical removal of the polyps can increase the chances for a woman to get pregnant.

Polycystic ovary syndrome (PCOS) is one of the most common causes of female infertility. It is a condition in which a woman’s body produces more androgens than normal. High levels of these hormones interfere with the development of ovarian follicles and release of eggs during ovulation.

Five causes of male infertility 

Male infertility refers to when a man has a small chance of making his female partner pregnant. Typically, It depends on the quality of his sperm cells. Examples of male infertility include:

• Libido is more commonly referred to as sex drive, libido describes a person’s desire to have sex. This can be negatively affected by stress, anxiety or depression.
• Erectile dysfunction otherwise known as impotence, erectile dysfunction is when a man is unable to develop or maintain an erection.
• Sperm count is a count of the number of sperm in a sample of semen. An important aspect of semen quality is the number or concentration of sperm cells in a given amount of semen.
• Sperm motility is the ability of sperm to move (swim) properly through the female reproductive tract to reach the egg. Sperm motility is the percentage of moving sperm cells in a sample of semen.

Low Testosterone levels may be responsible for infertility in some men.

How can obesity cause infertility?

Firstly, let’s define obesity. Obesity refers to abnormal or excessive fat accumulation that presents a risk to health. Adults are generally considered obese if they have a body mass index (BMI) of 30 or above. 

Researchers found obese women were much less likely to conceive within one year of stopping contraception than women in a healthy weight range.(3) Let’s look at this a little further.

Body fat affects the menstrual cycle through a range of hormonal mechanisms. The more excess weight and the more abdominal fat, the greater the risk of fertility difficulties. A fine hormonal balance regulates the menstrual cycle. Fatty tissue produces leptin, therefore overweight women have higher levels of this hormone. This can disrupt the hormone balance and lead to reduced fertility. 

Excessive abdominal fat is also linked to insulin resistance and decreased levels of sex hormone-binding globulin (SHBG), which is a protein that is involved in the regulation of two sex-hormones: androgen and oestrogen.(4) 

Like women, obesity can cause infertility in men. This is likely due to hormone problems, sexual dysfunction and other health conditions linked to obesity, such as type 2 diabetes and sleep apnoea. In fact, according to research weighing an extra 10kg may reduce male fertility by 10%.(5) 

It’s important to highlight that being underweight (BMI under 18.5) can also reduce a woman’s fertility by causing hormone imbalances that affect ovulation and the chance of getting pregnant. Compared to women in the healthy weight range, women who are underweight are also more likely to take more than a year to get pregnant. Additionally, a low BMI could reduce total sperm count and sperm concentration in men.(6,7)