When it comes to fertility, the numbers can tell a bigger story than you think. Here’s how BMI affects your chances of conceiving and what to do if it’s working against you

In 2022, the Australian Institute of Health and Welfare reported that 66 per cent ofAustralians over 18 were overweight—around 13 million adults—with 32 per cent considered obese. While these numbers are often linked to chronic disease, they also highlight a quieter concern: fertility. From BMI’s impact to hormonal conditions and emerging treatments, Fertility SA’s Dr Michelle Wellman explains why weight plays a bigger role in falling pregnant than many realise.

Understanding BMI

BMI stands for body mass index–one of the most common methods for measuring the percentage of body fat an individual has using height and weight. While Dr Wellman statest here are “more accurate” ways to measure health (think muscle mass tests and bone density scans), yourBMI is one of the universally understood.

A BMI of 18.5 to 25 is considered healthy for young to middle-aged adults.“Where there is a high indication of risk, further blood tests such as insulin, HbA1C and fasting blood sugar levels and lipids may be helpful to assess metabolic health,” Dr Wellman continues. This could apply to individuals whose BMI is reported to be between 25 and 28 (considered mildly overweight). “However, there is good evidence that the fertility and pregnancy outcomes are poorer in those living with obesity (BMI >30).”

Impact

There are several ways your BMI can impact your overall fertility. The feedback loop between your ovaries, brain and pituitary gland–responsible for producing and releasing hormones crucial to regulating the menstrual cycle – can be disrupted in individuals with a higher body mass, impacting estrogen and progesterone levels and impairing ovulation. According to Dr Wellman, increased levels of inflammation, which are related to obesity, “may also have a direct effect on the metabolism of the eggs and the way they mature.”

Obesity has a similar effect on sperm function and count, which is associated with increased temperature in the testes, overall inflammation and resulting hormonal changes (think decreased testosterone levels or a spike in estrogen production. A high BMI also increases the risk of sperm DNA damage, which impacts the quality of the cell and the chance of conceiving naturally.

During pregnancy, Dr Wellman says a high BMI runs the risk of miscarriage, potential abnormalities (such as spina-bifida), gestational diabetes and increased complications during operative delivery (caesarean section) and natural births, like shoulder dystocia. “A large number of studies have shown that weight loss before conception can reduce these risks,” she says.

“However… most birthing/obstetric units are experienced with assisting with pregnancy and birth for women living with obesity or [being] overweight.”

For both men and women, Dr Wellman says diet plays a “large part” in managing fertility, and individuals may want to engage in additional support from a dietitian, nutritionist, psychologist or exercise physiologist.

Extreme BMIs

On the other end of the scale, individuals with low body fat have similar issues regarding ovulation to those who are considered obese. Individuals whose BMI falls under 18.5 or who exercise at high levels are at risk of disrupting the hormones released from the pituitary gland, ceasing ovulation from occurring. “It may take some time–in some cases, several years–for the system to go back to normal, even once exercise has reduced and the fat mass has increased,” Dr Wellman says. “In some situations, injections of the pituitary hormones are required to induce ovulation.”

Fertility problems for men engaging in excessive training are less common. It may result in reduced testosterone, but “this is also likely to affect performance and therefore may be self-managed.” Tight athletic clothing can also increase heat around the testes, potentially impacting sperm maturation. However, the use of androgenic steroids is linked to cases of subfertility, and Dr Wellman says men who engage in this practice often present with “very low or absent sperm counts.

Hormonal imbalances

Hormonal imbalances significantly impact individuals’ ability to lose or gain weight. For women, the most common hormonal disruptions include Polycystic Ovary Syndrome(PCOS), whichThe Medical Journal of Australia found affects approximately one in 10Australian women. “The relationship between weight and PCOS is very complex,” explainsDr Wellman. Not all women whose PCOS has impacted their menstrual cycle will present as overweight. However, increasing weight does reduce and sometimes ceases the ability to ovulate.“

Conversely, small amounts of weight loss (approximately five per cent) can restore ovulation in some women,” Dr Wellman continues. “There is no ‘PCOS diet’ and weight loss is always difficult, but small amounts of weight loss can make a big difference with fertility.”Other conditions that disrupt hormone production include thyroid diseases(hyper/hypothyroidism), diabetes and reduced levels of testosterone. Further, any testosterone–prescribed or otherwise–will affect sperm maturation and fertility.

Before conceiving, Dr Wellman advises seeing your GP to optimise these underlying conditions, including addressing your current lifestyle habits. At least 30 to 40 minutes of moderate-intensity daily exercise and a balanced diet are key.

Next steps

Knowing when it’s time to reach out to fertility experts is crucial to addressing underlying issues and improving the chances of conception. “Often, there will be some embarrassment or self-blame,” Dr Wellman explains. “But one of the most common obstacles is delay in seeking help.” This, when coupled with older eggs or sperm, can increase the difficulty of falling pregnant.

For women, the most common issue is irregular periods. Addressing lifestyle factors is the first port of call and switching to a meal plan aligned with the Australian dietary guidelines is recommended.“

There are several newer medications that may be effective in assisting with weight loss, such as liraglutide, semaglutide and tirzepatide,” adds Dr Wellman. “There are likely to be several more available over the next few years. However, these need to be discontinued weeks to months before conception due to the potential risk of fetal abnormalities.”

Bariatric (weight-loss) surgery is another avenue that may be suggested, which can help improve the chances of conception and reduce the risk to mother and baby throughout pregnancy and birth. For those considering this route, Dr Wellman says it is generally recommended that conception is delayed for 12 months post-surgery.

What’s more, Dr Wellman says recommended supplements such as Folic Acid help reduce the risk of neural tube defects (such as spina bifida) when conceiving, and an Iodine supplement can help develop the baby’s nervous system. However, it is important to engage professionals throughout your fertility journey and follow their recommendations for your specific concerns and conditions.

Fertility SA
Level 9/431 King William Street, Adelaide
08 8100 2900
fertilitysa.com.au