Low, high or a bit either side – understanding BMI, for fertility and IVF

Words by Jessie Day, Editor

The ideal BMI for fertility – and in pregnancy – is a regular search these days. We know more than ever before about optimising our TTC (trying to conceive) game plan, and one thing that pops up is weight, BMI (body mass index) and how these affect our chances of conception, and a healthy pregnancy. 

BMI and fertility – where are we at?

For an at-a-glance ideal, current preconception guidance suggests we aim for a BMI between 18.5 to 24.9 – also known as the ‘healthy range’, within BMI guidelines. Whatever your gender, optimising can be helpful, and supports safe conception whether it’s assisted or natural. 

It’s never as simple, however, as being told ‘just lose the weight’. Your wellbeing as patient and parent-to-be should always be top priority, with the best clinics and fertility teams helping you understand where BMI fits into the overall picture. And, crucially, supporting you in taking next steps, to move forward with your plan for safe, healthy conception. 

Part of our go-to network for cutting-edge expertise, and patient-first care and support, TFP Fertility UK help patients create a completely unique plan, built just for them. From low or high BMI to specific health concerns and underlying conditions, they are prepped to make your experience the absolute best it can be. 

We caught up with TFP Fertility UK Medical Director Dr James Hopkisson for a quick-fire Q&A on BMI and fertility, putting the most common questions into the spotlight. 

Over to Dr Hopkisson. We’ve also just wrapped a full deep-dive Q&A on the ideal BMI for fertility, which you can catch here.

Where do you start with patients, and BMI?

First of all, we want to reassure people that high or low, BMI and fertility is a very common, but often sensitive topic. The team at TFP Fertility UK, whether you’re working with myself or one of my colleagues, one hundred per cent understand this, and that it can trigger all sorts of responses, during consultation. 

This is normal, and perfectly ok! Our goal is to build a plan completely dedicated to you, and your situation. Looking more specifically at BMI, we’ll always prioritise safe and effective treatment. Like many clinics we do have some restrictions in place for the BMI threshold we’re able to work with, but we’ll always explain these, and support actionable, positive next steps for your family-building journey.

What do we mean by BMI?

BMI – or body mass index – is a simple calculation. The NHS provides a calculator and supporting information, if you need to work yours out quickly. But essentially, it uses your weight and height to provide a measure, setting out whether your weight is within a healthy range. It’s not specific to conception, and is used by healthcare professionals in all sorts of settings to establish your baseline medical profile. 

Is BMI a reliable measure, when looking at fertility?

BMI is something that we recognise, and it is important. It’s not a perfect measurement, and in some cases there will be issues with calculating. For example for people who are more muscular or athletic, their muscle will weigh more than fatty tissue.

So, we recognise it’s not a perfect measurement, but it’s the best that we have to gauge effects on health, whether that’s fertility or cardiovascular health, diabetes, blood pressure, and so on.

How can BMI impact fertility (natural or assisted)?

Let’s start with BMI on the higher end of the chart (keep reading for low BMI and fertility, however, because this is also significant). 

Again, your safety and effective treatment are of paramount importance to us. This isn’t about a judgement of weight or lifestyle, it’s about your wellbeing, and ensuring we’re not putting you or a pregnancy at unnecessary risk. 

With a higher BMI (above 25), your risk can increase for: 

At conception (natural or assisted) 

• poorer rates of ovulation, and response to fertility treatment drugs like letrozole and clomid 
• increased need for dosing with the drug gonadotropin (which carries associated risk factors, including multiple pregnancy and OHSS) 
• difficulty monitoring your follicle development, and egg retrieval 
• complications during/following sedation, depending on your treatment plan 

During pregnancy (and after)

• miscarriage
• maternal health problems, such as gestational diabetes, high blood pressure and preeclampsia
• birth intervention
• postpartum complications, such as haemorrhage and venous thromboembolism

There are many factors that come into play. But top priority for us is your safety, and a treatment plan and journey that sets you up for successful, safe conception.

A note on dosages

When working out medication dosages for an IVF program, it’s often more difficult to tailor dosing to the individual, for people who are overweight, and response can be far more unpredictable. We also know that people who are overweight will require more medication to get the same yield of eggs in their IVF program.

As practitioners, our duty is to help. Patients want clarity, support and actionable next steps – which I very much aim for, in every consultation. 

So for a bottom line on the ideal BMI for fertility, a high measurement (above 25) is associated with: 

• a decrease in the effectiveness of fertility treatment, but also 
• an increase in risk, both for your fertility procedure, and your pregnancy going forward

When we’re using medical intervention to help you have a baby, we want to achieve the best outcome, but also the safest. It’s not about saying ‘come back and see me when you’ve lost weight’ – which sadly I do hear, time and again. 

It’s about saying, ‘we will work through this’.

What’s the ‘ideal’ BMI for fertility?

A BMI between 18.5 and 25 is ideal. With a BMI of 25 to 30, we do see good pregnancy rates, which is why NICE guidelines suggest that the NHS should fund IVF up to a BMI of 30.

Many fertility clinics will implement their own restrictions for treatment, using BMI. At TFP Fertility UK, most of our UK-wide units are able to treat patients with a BMI up to 35, depending on sedation risks and the practical aspect of egg retrieval, which can be impacted by a higher BMI. 

What about a low BMI and fertility?

A BMI below 18.5 can create issues for fertility, which are often related to hormonal balance. Low body weight can prevent the necessary signalling needed for regular, effective ovulation, so supporting that is important. 

Alongside this, a low BMI can create more risk of hyperemesis during pregnancy (where you’re unable to tolerate solids and fluids during pregnancy), due to lower metabolic reserve.

Does BMI affect male fertility?

Yes! 

Focusing on sperm, a number of studies have shown that men who are markedly overweight may have lower counts, lower motility, and lower spontaneous conception rates, as well as having problems in assisted conception.

NICE guidelines do recognize that changing your lifestyle will affect the quality of your sperm, if you have a higher BMI. And we also know that in terms of DNA damage, lifestyle for men is very important, when looking at fertility.

Unfortunately, the vast majority of restrictions in fertility and assisted conception are more targeted at women. Of course, it’s often a woman who is undergoing an invasive procedure, and then pregnancy. But studies show that for optimised fertility, the male BMI is significant.

What’s TFP Fertility UK’s BMI limit for IVF?

I’m happy for anyone to talk to me and get advice about fertility, from assessments to treatment, right at the outset. Getting good quality advice at the outset should be the priority, and next steps designed to fit your individual profile and journey.

When it comes to actual treatment, most units will set an upper threshold for BMI of around 35. 

Keen to dive deeper? Watch the full Q&A interview with Dr James Hopkisson, covering the latest findings and approaches for BMI for conception and pregnancy. Once you’re caught up, touch base with the team at TFP Fertility UK for support and next steps, specific to your unique journey.