Fertility treatments and obesity are closely linked, as excess body weight significantly affects reproductive health and the success of assisted conception. Obesity, defined as a body mass index (BMI) of 30 kg/m² or above, disrupts hormonal balance, impairs ovulation, and reduces sperm quality in men. Women with obesity often require higher medication doses during fertility treatment and may experience lower success rates. However, even modest weight loss of 5–10% can restore ovulatory function and improve outcomes. Understanding how obesity impacts fertility treatment helps couples make informed decisions and optimise their chances of conception through evidence-based weight management and tailored clinical approaches.

How Obesity Affects Fertility and Conception

Obesity, defined as a body mass index (BMI) of 30 kg/m² or above, has a significant impact on reproductive health in both women and men. In women, excess adipose tissue disrupts the delicate hormonal balance required for regular ovulation. Adipocytes (fat cells) produce oestrogen, and elevated levels can interfere with the hypothalamic-pituitary-ovarian axis, leading to anovulation or irregular menstrual cycles. Obesity is also commonly associated with polycystic ovary syndrome (PCOS), a leading cause of anovulatory infertility. While PCOS occurs across all BMI ranges, obesity worsens key features such as insulin resistance and hyperandrogenism, making ovulation less likely.

The metabolic consequences of obesity extend beyond ovulation. Insulin resistance, commonly seen in individuals with elevated BMI, may affect the quality of oocytes (eggs) and the receptivity of the endometrium (uterine lining). Research suggests that women with obesity may experience reduced oocyte quality, impaired embryo development, and lower implantation rates, though these findings can be influenced by other factors such as age and underlying conditions like PCOS. Obesity is also associated with an increased risk of miscarriage in both natural conception and assisted reproduction.

In men, obesity contributes to subfertility through multiple mechanisms. Excess body fat leads to increased conversion of testosterone to oestrogen via the enzyme aromatase, resulting in reduced testosterone levels and impaired spermatogenesis. Men with obesity may present with lower sperm concentration, reduced motility, and increased DNA fragmentation in sperm. Elevated scrotal temperature due to increased adipose tissue in the suprapubic region may further compromise sperm production.

Weight loss in both partners can improve natural conception rates and optimise outcomes from fertility treatment. Even modest weight reduction of 5–10% of total body weight can restore ovulatory function in many women with anovulatory infertility.

Key fertility impacts of obesity include:

• Disrupted ovulation and menstrual irregularity

• Reduced oocyte and sperm quality

• Hormonal imbalances affecting conception

• Increased miscarriage risk

• Lower natural conception rates in both partners

Further information:

• NICE CG156: Fertility problems – assessment and treatment

• HFEA patient information: Weight and fertility

• NHS: Infertility – causes and lifestyle factors

Impact of Obesity on Fertility Treatment Success Rates

Evidence demonstrates that obesity can reduce the success rates of assisted reproductive technologies (ART), including in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI). Women with a BMI above 30 kg/m² often require higher doses of gonadotrophin medications to stimulate ovarian follicle development, yet may produce fewer mature oocytes. The ovarian response to controlled ovarian hyperstimulation can be suboptimal, with research showing a relationship between increasing BMI and reduced oocyte yield.

Implantation rates and clinical pregnancy rates may be lower in women with obesity undergoing IVF. Systematic reviews have found that increasing BMI is associated with reduced live birth rates per treatment cycle. The mechanisms underlying this reduction may include impaired endometrial receptivity, altered gene expression in the endometrium, and chronic low-grade inflammation associated with excess adiposity. However, outcomes vary between individuals, and many women with obesity achieve successful pregnancies through fertility treatment.

Women with elevated BMI may face increased technical considerations during fertility treatment cycles. The increased pelvic adipose tissue can make transvaginal ultrasound monitoring and egg collection more challenging in some cases, and anaesthetic considerations become more important at higher BMI levels. It is important to note that the risk of ovarian hyperstimulation syndrome (OHSS) is primarily related to individual ovarian response and conditions such as PCOS, rather than BMI itself. Modern protocols using GnRH antagonists and agonist trigger strategies have significantly reduced OHSS risk across all BMI ranges.

Treatment outcome considerations:

• Higher gonadotrophin requirements with variable ovarian response

• Potentially lower oocyte retrieval numbers and quality

• Reduced implantation and live birth rates per cycle in some studies

• Increased anaesthetic and technical considerations at higher BMI

• Greater financial and emotional burden if multiple cycles are needed

Further information:

• HFEA: Fertility treatment outcomes by patient characteristics

• British Fertility Society guidance on ovarian stimulation protocols

Weight Management Before Starting Fertility Treatment

Achieving a healthier weight before commencing fertility treatment is one of the most evidence-based interventions to improve reproductive outcomes. NICE guidelines recommend that women with a BMI above 30 kg/m² should be informed that weight loss is likely to improve their chances of conception and should be offered support to lose weight. Even modest weight reduction of 5–10% of total body weight can restore ovulatory function in many women with anovulatory infertility, potentially enabling natural conception without the need for assisted reproduction.

A structured, multidisciplinary approach to weight management yields the best results. This typically involves dietary modification, increased physical activity, and behavioural support. Referral to specialist weight management services (Tier 2 or Tier 3 services), dietitians, or evidence-based commercial weight loss programmes may be appropriate. The focus should be on sustainable lifestyle changes rather than rapid weight loss, which can be counterproductive and difficult to maintain. A balanced, calorie-controlled diet (typically a deficit of 500–600 kcal per day) combined with regular moderate-intensity physical activity (at least 150 minutes per week, plus muscle-strengthening activities on two or more days per week) forms the foundation of effective weight management.

For individuals with a BMI of 40 kg/m² or above, or 35 kg/m² or above with significant obesity-related comorbidities, who have not achieved adequate weight loss through lifestyle measures, bariatric surgery may be considered. Procedures such as gastric bypass or sleeve gastrectomy can result in substantial, sustained weight loss and improvement in metabolic parameters. Fertility treatment is typically deferred for 12–18 months post-surgery to allow for weight stabilisation, nutritional optimisation, and recovery. Effective contraception is essential during this period.

Weight-loss medications may be considered in some cases under specialist supervision, in line with NICE guidance (for example, orlistat or semaglutide where criteria are met). If pharmacological treatment is used, it is essential that effective contraception is in place, as these medications are not suitable during pregnancy. Women planning to conceive should discuss stopping weight-loss medication with their clinician and allow an appropriate washout period before attempting conception.

Weight management strategies include:

• Structured dietary plans with calorie reduction (typically 500–600 kcal deficit)

• Regular physical activity: at least 150 minutes moderate-intensity exercise per week, plus strength training on two or more days

• Behavioural therapy and psychological support

• Involvement of specialist weight management services (Tier 2/3)

• Consideration of pharmacological or surgical options in selected cases under specialist guidance

• Regular monitoring and adjustment of interventions

Further information:

• NICE CG189: Obesity – identification, assessment and management

• UK Chief Medical Officers' Physical Activity Guidelines (2019)

• NHS: Weight management services and support

Fertility Treatment Options for Patients with Obesity

Despite the challenges posed by obesity, various fertility treatment options remain available, with individualised approaches based on the underlying cause of infertility and patient circumstances. For women with anovulatory infertility and PCOS, ovulation induction with oral medications represents a first-line approach. Clomifene citrate is licensed in the UK for anovulatory infertility. Letrozole (an aromatase inhibitor) has demonstrated superior ovulation and live birth rates compared to clomifene in women with PCOS and is now recommended as first-line treatment by international guidelines; however, its use for ovulation induction is off-label in the UK and should be prescribed under specialist supervision with informed consent.

When oral ovulation induction is unsuccessful, gonadotrophin injections (follicle-stimulating hormone, FSH) may be used to stimulate follicular development, typically combined with intrauterine insemination (IUI) or timed intercourse. Women with PCOS, including those with obesity, require careful monitoring during gonadotrophin therapy due to increased risks of multiple follicle development. A low-dose step-up protocol is recommended to minimise the risk of multiple pregnancy and ovarian hyperstimulation syndrome (OHSS), though higher absolute doses may be required in women with higher BMI. More frequent ultrasound monitoring is often necessary to ensure safe treatment.

For couples requiring IVF or ICSI, treatment protocols may be modified to optimise outcomes and safety. GnRH antagonist protocols offer greater flexibility and, when combined with GnRH agonist trigger, significantly reduce the risk of OHSS in high-risk patients. Embryo freezing strategies, where all embryos are cryopreserved and transferred in subsequent frozen embryo transfer (FET) cycles, may be considered to reduce OHSS risk and allow optimal endometrial preparation without the metabolic effects of ovarian stimulation, though evidence for improved implantation rates in the context of obesity is mixed and outcomes should be discussed on an individual basis.

Available treatment modalities:

• Ovulation induction with clomifene citrate (licensed) or letrozole (off-label; specialist use with informed consent)

• Gonadotrophin therapy with low-dose step-up protocols and careful monitoring

• Intrauterine insemination (IUI) with or without ovarian stimulation

• In vitro fertilisation (IVF) with protocol modifications (e.g., GnRH antagonist)

• Intracytoplasmic sperm injection (ICSI) for male factor infertility

• Freeze-all strategies with subsequent frozen embryo transfer to reduce OHSS risk

Further information:

• BNF: Clomifene citrate and gonadotrophin preparations – indications, monitoring, and adverse effects

• International evidence-based guideline for PCOS (ESHRE/ASRM 2023)

• HFEA patient information: Treatment options and OHSS

If you experience any suspected side effects from fertility medications, you can report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

NHS Guidelines on BMI and Fertility Services

NICE Fertility Guidelines (CG156) recommend that women with a BMI outside the range of 19–30 kg/m² should be informed that achieving a BMI within this range is likely to improve their chances of conception. The guidelines advise that women with a BMI above 30 kg/m² should be offered support to lose weight. However, NICE does not mandate specific BMI cut-offs for accessing fertility treatment; these decisions are made by local commissioners.

In England, fertility services are commissioned by Integrated Care Boards (ICBs), which have replaced Clinical Commissioning Groups. Many ICBs have implemented BMI thresholds for accessing NHS-funded fertility treatment, with most requiring women to have a BMI below 30 kg/m² to qualify for NHS-funded IVF. Some areas apply a threshold of BMI below 35 kg/m². These policies are based on evidence demonstrating reduced treatment success rates and increased health risks at higher BMIs, as well as considerations of resource allocation and cost-effectiveness. The variation in local policies means that eligibility criteria differ across the country, and couples are advised to contact their local ICB or fertility clinic to understand specific criteria in their area.

For male partners, BMI is less commonly included in eligibility criteria, though some ICBs do consider male BMI. The focus on female BMI relates primarily to the direct impact on pregnancy outcomes and obstetric risks, though the effect of male obesity on sperm quality is increasingly recognised.

Private fertility treatment may be available to individuals who do not meet NHS eligibility criteria; however, many private clinics also apply BMI limits for clinical safety reasons, particularly related to anaesthetic risk and procedural considerations. It is important to discuss individual circumstances with the treating clinic.

Key policy points:

• NICE recommends optimal BMI range of 19–30 kg/m² and advises weight management support

• Local ICB policies vary regarding BMI thresholds for NHS-funded treatment

• Most ICBs require female BMI below 30 kg/m² for funded IVF; some set the threshold at BMI below 35 kg/m²

• Weight management support should be offered before treatment

• Private clinics may also apply BMI limits for safety and anaesthetic reasons

Further information:

• NICE CG156: Fertility problems – assessment and treatment

• HFEA: NHS funding and eligibility – regional variation

• Your local ICB fertility policy (contact your ICB or GP for details)

Supporting Healthy Pregnancy Outcomes After Fertility Treatment

Achieving pregnancy through fertility treatment is only the first step; supporting a healthy pregnancy and delivery is equally important, particularly for women with obesity who face increased obstetric risks. Obesity in pregnancy is associated with gestational diabetes, pre-eclampsia, venous thromboembolism, caesarean delivery, and macrosomia (large baby). Women who conceive through ART with an elevated BMI require enhanced antenatal surveillance and multidisciplinary care throughout pregnancy.

Preconception and early pregnancy optimisation is crucial. Women with obesity planning fertility treatment or pregnancy should commence folic acid supplementation at a higher dose of 5 mg daily (rather than the standard 400 micrograms) to reduce the risk of neural tube defects. Vitamin D supplementation of 10 micrograms daily is also recommended, as obesity is associated with vitamin D deficiency. Women with a BMI of 30 kg/m² or above should be offered testing for undiagnosed diabetes at the first antenatal appointment (using HbA1c or fasting plasma glucose). If this test is normal, a 75 g oral glucose tolerance test (OGTT) should be offered at 24–28 weeks of pregnancy to screen for gestational diabetes.

Ongoing weight management during pregnancy requires a balanced approach. While weight loss is not recommended during pregnancy, women should be supported to eat healthily and remain physically active (unless contraindicated). NICE does not set specific gestational weight-gain targets; instead, the focus is on healthy eating, regular physical activity, and routine weight monitoring at antenatal appointments. Referral to specialist obstetric services and consultant-led care is recommended for women with a BMI of 35 kg/m² or above, according to local protocols. Women with a BMI of 40 kg/m² or above should be offered an antenatal anaesthetic assessment in the third trimester.

Venous thromboembolism (VTE) risk assessment should be carried out at booking and repeated if circumstances change. Low-molecular-weight heparin may be indicated for VTE prophylaxis, depending on individual risk factors.

Pregnancy care considerations:

• High-dose folic acid (5 mg daily) and vitamin D (10 micrograms daily) supplementation

• Testing for undiagnosed diabetes at booking if BMI ≥30 kg/m²; OGTT at 24–28 weeks for women with risk factors

• Enhanced antenatal surveillance and consultant-led care (typically for BMI ≥35 kg/m²)

• VTE risk assessment and thromboprophylaxis with low-molecular-weight heparin if indicated

• Anaesthetic review in third trimester for women with BMI ≥40 kg/m²

• Postnatal weight management support and contraception counselling

You should contact your GP or midwife promptly if you experience symptoms such as severe headache, visual disturbances, upper abdominal pain, or reduced fetal movements, as these may indicate pregnancy complications to which you are at increased risk.

Further information:

• RCOG Green-top Guideline No. 72: Care of Women with Obesity in Pregnancy

• NICE NG3: Diabetes in pregnancy

• NICE PH27: Weight management before, during and after pregnancy

• NHS: Vitamins, supplements and nutrition in pregnancy

Frequently Asked Questions