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  • Writer's pictureIsobel Austin-Little


weight and fertility
can weight affect your fertility

Does weight/BMI affect fertility? Here's what we know about how being underweight or overweight impacts a persons ability to conceive.

According to the NHS, a healthy BMI is between 18.5 -24.9 kg, underweight is anything less than 18.5, overweight is a BMI between 25-29.9, and obese is anything over 30. In my practice, I feel more comfortable when a client's BMI is over 19/20. That being said, it is also important to note that BMI is not a determinant of fertility on its own; it needs to be considered in the context of that person, as BMI does not factor in variables such as general health, bone density and muscle mass. 

However, we know that being overweight, obese or underweight can impact a person's ability to conceive, and therefore, maintaining a healthy weight when trying for a baby is necessary to increase the chances of a successful conception but also for the health of the baby.

Let's take a look at why. 

What are the complications of being underweight when trying for a baby?

For most women, having a BMI under 18.5 indicates that you are underweight. However, having a low BMI may only sometimes cause problems with fertility. As a Fertility Nutritionist, what I am looking out for in a person with a low BMI is the following:

  • Is there an eating disorder?

  • Have they got stress affecting their appetite?

  • Is the person getting adequate nutrients in their diet?

  • Are there absorption issues?

  • Are they over-exercising / have insufficient caloric intake?

  • Are they unwell, or do they have a medical condition that needs support (diabetes or a thyroid condition)?

A low BMI can out you at risk of having low estrogen, resulting in a hormone imbalance that can impacting ovulation. Being underweight can also stop periods altogether, making it difficult to conceive. 

How does being overweight or obese impact fertility?

Disrupts Ovulation. Ovulation needs to take place for conception to occur. I have spoken a lot about the intricate dance that takes place between hormones during a woman's cycle, and it is impacted by leptin. Overweight and obese women have higher levels of leptin hormone, which is produced in the fatty tissue. Leptin can disrupt hormone balance and result in reduced fertility. 

We also know that excessive abdominal fat is linked to estrogen excess and insulin resistance. Insulin resistance occurs when your body's cells don't respond appropriately to the insulin produced, so the body has to produce more insulin to keep the blood sugar levels in normal range). Insulin resistance is associated with low Sex Hormone Binding Globulin, which is vital for regulating estrogen and androgens. This also increases the risk of irregular cycles and disrupted ovulation, which can negatively impact fertility. A scenario that is often observed in women with PCOS.

What about egg quality?

If ovulation does occur, that is not to say that egg quality is optimal. Egg maturation and metabolism are impaired in obese women, impacting egg quality, which can also affect a healthy conception. Research suggests that Obesity is associated with lower pregnancy rates in subfertile ovulatory women. Those women with a higher BMI have a 4% lower pregnancy rate. This indicates that egg quality could be compromised, impacting successful conception.

In women who undergo IVF, where ovulation is not required to conceive conception/IVF, success rates decrease in obese patients and reaching a BMI of 30 is often required before starting treatment. 

What about men, does being overweight affect their fertility?

A high BMI can also impact the quality and quantity of a man's sperm, contributing to DNA damage in sperm and infertility.

Obesity and pregnancy outcomes.

During pregnancy, maternal Obesity negatively impacts fetal development, and there is an increased risk of pregnancy complications such as gestational diabetes and preeclampsia. In terms of the child's health in later life, we know that maternal Obesity carries long-term risks for the newborn and an increased risk of being overweight as adults.

So, to summarise.

It is clear that establishing a healthy weight/BMI before conceiving and during pregnancy is essential in improving the chances of a successful conception and for the health of the mother and the offspring. What is clear is how BMI, whether high or low, can contribute to hormone imbalance, affecting fertility.

If you have stopped menstruating, seeking help and advice from your GP or a Registered Nutritionist is important. Ovulation is essential for fertility, but outside of a fertility setting, having adequate levels of estrogen and progesterone is critical for wider health, too. These two hormones work together like yin and yang, estrogen and progesterone, to promote the long-term health of the brain, bones, and cardiovascular system. For example, in the brain, Estradiol lifts you up by boosting serotonin, oxytocin, and dopamine. Progesterone calms you down by acting like GABA in your brain. 

You can check your BMI here if you are concerned about your weight. If you need support investigating your health and guidance on reaching your desired BMI/weight to optimise your chances of a healthy conception and pregnancy, then get in touch today here. In the meantine, checkout my 6 simple steps to a healthy conception.

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