BMI and fertility

What is BMI?

Body mass index (BMI) is a measure of body fat based on your weight in relation to your height, and applies to most adult men and women aged 20 and over.

What is my BMI?

You can work out your BMI by using a BMI calculator.

Does elevated BMI effect my chances of having a baby?

National guidelines on obesity and infertility are very clear; your weight can affect your fertility. Guidelines indicate that your chances of getting pregnant with IVF are better if your BMI is between 19 and 30. A BMI over 30 has a significant negative effect on IVF pregnancy success rates. IVF success rates are most favourable in patients with low and normal BMIs. With increasing BMI, there is a steady and significant decrease in implantation rate, clinical pregnancy rate, and live birth rate and an increase in pregnancy loss.
Another issue related to body weight and IVF is safety for the patient at the egg retrieval. When a woman is significantly overweight, the ovaries are usually pushed up “high” away from the top of the vagina by the extra fatty tissue that is in the pelvis. At the time of IVF, the needle is pushed in vaginally to reach the eggs in the ovaries. If the ovaries are too high, we can not safely get the needle into the follicles to get the eggs out. Another problem is that the ultrasound images become very “fuzzy” from the extra tissue between the probe and the ovary. Therefore, it is often difficult to clearly visualize the ovaries and the egg-containing follicles. This makes it difficult to properly measure the follicles in the ovaries, and can also make it hard to be sure where the needle tip is located at all times during the egg retrieval procedure (a potential safety issue).

Why does high BMI lower my chances of success?

The mechanism of the negative effect of BMI on IVF outcome is unclear. Recent studies have suggested decreased egg quality in overweight patients. Alternatively, altered endometrial receptivity (the ability of the woman’s womb to ‘allow’ implantation) may play a role. Increased BMI also effects embryo development negatively impacting blastocyst formation rate in IVF (less embryos will grow to the day-5 blastocyst stage.

What about risks in pregnancy with a higher BMI?

Women who were overweight or obese (BMI ⩾25)  have significantly lower clinical pregnancy and live-birth rates and significantly higher miscarriage rate  compared with women with a BMI < 25 following treatment. Increasing BMI is associated with increased incidence of pre-eclampsia, gestational hypertension, high birth weight in babies, induction of labour and caesarean delivery.

How about BMI in men undergoing fertility treatment?

Men with a BMI of 30 or over are more likely to have reduced fertility and a reduced likelihood of becoming a father. A raised male BMI is associated with a significant reduction in clinical pregnancy rate and live birth rate in patients having IVF.

So what should does my BMI need to be for treatment at CRGW?

Due to effect on successful outcome and risks both during fertility treatment and any pregnancy resulting from fertility treatment at CRGW, we set our BMI limits for female patients receiving treatment as:

Treatment BMI required
Clomid <40
IUI <40
IVF / ICSI <35