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This information is designed to help you have the best chance to get pregnant and have a healthy baby. Your healthcare professionals will not judge you for being overweight or underweight and will support you in optimising your weight.
A healthy diet is very important for women who are trying for a baby. Most women who are underweight, overweight or obese manage to conceive naturally and deliver healthy babies. However, being overweight or underweight can have adverse effects on fertility.
Both maternal and paternal diet and health is now known to have an impact on the health of future generations (your children’s future children).
What is BMI?
Your body mass index (BMI) is used to calculate whether your weight is healthy. The body mass index is what healthcare professionals use to guide your fertility management. It is your weight in kilograms divided by your height in metres squared (kg/m²).
WHO* Classification of BMI
| Under weight | <18.5 kg/m² |
| Normal BMI | 18.5- 24.9 kg/m² |
| Overweight | 25.0 – 29.9 kg/m² |
| Moderate obesity | 30.0 – 34.9 kg/m² |
| Severe obesity | 35.0 – 39.9 kg/m² |
| Very Severe obesity | 40.0 kg/m² and above |
What are the effects of low and high BMI on Fertility?
Being either overweight or underweight can reduce fertility
| High BMI= higher than30 kg/m² | Low BMI= lower than18.5 kg/m² |
| Irregular menstrual cycle | Irregular menstrual cycle |
| No or irregular egg release from ovaries (anovulation or irregular ovulation) |
No or irregular egg release from ovaries (anovulation or irregular ovulation) |
| Reduced pregnancy rate | Not enough body fat for fertility or to support a pregnancy |
| Reduced IVF success rate | Reduced IVF success rate |
| Increased risk for fertility surgical treatment such as egg collection, pelvic surgery | Reduced quality eggs and sperm due to low vitamin and mineral stores |
What are the risks of pregnancy with obesity?
- Miscarriage (more than 1in 4), stillbirths
- High blood pressure, heart disease and blood clots during pregnancy
- Diabetes in pregnancy
- Birth defects such as heart defects, neural tube defects
- High birth weight infants and risk of developing diabetes in later life
- Caesarean section
- Postpartum haemorrhage (heavy bleeding after giving birth)
If you have a BMI of 30 kg/m²or above, remember to start taking 5 mg of folic acid every day(request on prescription from your GP)and 10 mcg of Vitamin D every day for at least a month before you start trying to conceive, ideally for three months before starting.
What are the benefits of reducing weight if overweight/obese?
Weight loss of 5-10% can achieve a 30% reduction in internal fat stores and significantly improve overall health including fertility outcome. A woman who is overweight and is planning a pregnancy would be advised to move towards optimal weight before conceiving in order to:
1) Reduce miscarriage risk
2) Improve ovulation
3) Improve pregnancy rate
4) Reduce high blood pressure
5) Reduce chances of developing diabetes, reduce insulin resistance
6) Reduce heart disease
7) Improve self-esteem and well-being
8) Improve outcomes of fertility treatments
Is high BMI associated with polycystic ovarian syndrome(PCOS)?
PCOS is a very common condition in young women (about 8%–10%). Not all women with PCOS are overweight or obese, but many women with PCOS have signs of insulin resistance, with or without obesity. A reduced calorie diet and increasing exercise may lead to weight loss, regular menstrual cycles and ovulation, however some women may require medications to decrease insulin resistance.
How PCOS is treated?
- Reduced calorie diet if overweight/obese
- Exercise including both cardio-vascular and weight training exercise as well as walking a minimum of 30 minutes a day
- Medications to decrease insulin resistance if required such as Metformin
- Assistance of a reproductive endocrinologist and infertility specialist, if required
How can you achieve a normal/desired weight if overweight/obese?
- Making improvements in lifestyle
- Changes in diet, reducing calorie intake and choosing low fat and low sugar foods, wholegrain carbohydrates, 5 a day of vegetables and fruit, lower fat dairy foods 2-3 times a day, lower fat protein choices twice a day and oily fish twice a week
- Expert advice from dietitian, referral via your consultant or GP if required
- Exercise regime for a structured weight loss programme (aim to lose up to 1 kg or 2 lbs a week), aim for a minimum of 150 minutes of exercise per week
- Medication, discuss with your GP if required
- Bariatric Surgery, discuss with your GP if required
How can you achieve a normal/desired weight if underweight?
- Making improvements in lifestyle
- Change in diet, increasing calorie intake and continuing wholegrain carbohydrates. Aim for 5 a day of vegetables and fruit, choose full fat dairy foods 2-3 times a day, increase portions of protein foods at least twice a day and have oily fish twice a week
- Ensure you are eating enough to meet your exercise levels for the macronutrients needed for fertility(protein, fat and carbohydrates)
- Expert advice from dietitian
- Reduce exercise to a maximum of 60 minutes per day1
- Switch some exercise sessions to lower intensity exercises such as pilates/yoga
What are the criteria for fertility treatment?
If you are overweight or underweight, you may not qualify for NHS funded IVF treatment as it is likely to be less effective.
- Your BMI should be more than 19 kg/m² or less than 29.9 kg/m² for at least 6 months.
- If your BMI is more than 30 kg/m², it is advised that you lose weight to a BMI of less than (or closer to) 30 kg/m² before treatment. Ovulation induction is then more likely to work as is any surgical treatment to improve fertility, such as for pelvic endometriosis or fibroids in the uterus.
- If your BMI is more than 34.9 kg/m², it is a Trust Policy that fallopian tubal patency test cannot be requested because of difficulties with access in performing the test.
Does obesity affect male fertility?
Obesity can cause the temperature of the scrotum to increase, lead to erection problems and change the levels of male hormones that are produced by the body. This can all affect the quality of the sperm, which can reduce the chances of pregnancy, reduce the chances of success with assisted reproduction techniques and can raise the risk of miscarriage.
Where can you get help?
- Your GP, a visit to a healthcare professional before becoming pregnant can help identify other health problems related to body weight that can impact on pregnancy, such as polycystic ovary syndrome (PCOS), thyroid disease, diabetes, eating disorders and inflammatory states such as inflammatory bowel disease (IBD). Ask what services are available in your area to help with weight loss.
- Reproductive Endocrinologists or Infertility Specialist in Hospital, referral via your GP
- Accredited Specialist Dietitian or Fertility Nutritionist, referral via your GP
- Lifestyle Weight Management Providers, see the link under ‘other information sources’
- Self-help Groups
- Complimentary therapies such as acupuncture, stress management services
Other informative sources
Physical activity guidelines for adults aged 19 to 64 - NHS
Healthy ways to gain weight - NHS
Food Facts - The association of UK dietitian
Contact us
If you have any queries relating to this information, please contact the Gynaecology service.
About this information
Service:
Gynaecology
Reference:
L/125
Approval date:
1 October 2023
Review date:
1 January 2026
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Important note
This page provides general information only. It is developed by clinical staff and is reviewed regularly every 3 years for accuracy. For personal advice about your health, or if you have any concerns, please speak to your doctor.