Weight, fertility and pregnancy health

Please note: This page is about your weight before getting pregnant. If you are already pregnant and want to lose weight, it is important to talk to your doctor or a dietitian first, before making any lifestyle changes.

Most people know that being overweight or obese increases the risk of health problems such as heart disease and diabetes. Many are unaware that an unhealthy weight can also reduce fertility and the chance of having a healthy baby. If you are trying to conceive, or plan to start, the closer you are to a healthy weight the greater your chance of getting pregnant and having a healthy baby.

Understanding body mass index (BMI)#

Body mass index (BMI) is a number based on your height and weight. As a general guide:

• a healthy BMI is between 18.5 and 24.9
• a BMI between 25 and 29 is considered overweight
• a BMI of 30 or more indicates obesity
• a BMI under 18.5 is considered underweight

The ideal weight for conception varies from person to person and depends partly on how tall you are.

Weight and fertility in women#

Being an unhealthy weight can affect a woman’s fertility by causing:

• hormonal imbalances
• problems with ovulation (releasing an egg from the ovaries)
• menstrual disorders

It can also make assisted reproduction more difficult. Obesity is also associated with polycystic ovary syndrome (PCOS), a common cause of reduced fertility or infertility.

Being underweight can also reduce fertility, by causing hormone imbalances that affect ovulation. Compared with women in the healthy weight range, women who are underweight are more likely to take more than a year to get pregnant. If you are underweight and trying to conceive, a dietitian can work with you to adjust your diet and help you reach a healthier weight.

Weight and fertility in men#

Being very overweight or obese can also reduce a man’s fertility through:

• hormone problems
• problems with erections
• other health conditions linked to obesity

Weight loss in men who are overweight or obese can improve total sperm count and quality. Because sperm take about three months to develop, reaching a healthier weight at least three months before conception gives the best chance of producing healthy sperm and conceiving.

Weight and pregnancy health#

Obesity, and excess weight gain during pregnancy, is linked with a number of pregnancy complications, including an increased risk of:

• miscarriage
• high blood pressure and pre-eclampsia
• gestational diabetes
• caesarean birth
• stillbirth

Babies born to mothers who have obesity also have an increased risk of obesity in childhood and adulthood.

The good news#

While these facts can seem daunting, there is good news. In women who live with obesity, even a small weight loss improves fertility and pregnancy health. Diet and lifestyle changes that limit excessive weight gain during pregnancy can also improve health outcomes for both mother and baby.

Getting closer to a healthy weight before conception increases the chance of the baby being healthy at birth and into adulthood. This is also important when using assisted reproductive technologies (ART) such as in-vitro fertilisation (IVF), ovulation induction and intracytoplasmic sperm injection (ICSI). Studies have found that obesity reduces the rate of pregnancy and live births in women using ART and increases the risk of miscarriage, while weight loss through diet and lifestyle changes significantly improves these outcomes.

Getting ready for pregnancy#

If you are planning to get pregnant, starting a healthy eating and exercise plan now improves your chances of conceiving and having a healthy baby. Making these changes will benefit your overall health, even if they do not lead to weight loss. Begin before you start trying to conceive, and keep them up through pregnancy and beyond. Ask your doctor about any dietary supplements you may need, such as folic acid and iodine.

Try these steps to a healthier weight:

• Follow a healthy, balanced diet drawn from the main food groups: vegetables and legumes; fruit; wholegrain breads and cereals; milk, yoghurt, cheese and alternatives; and lean meat, poultry, fish, eggs, seeds and nuts.
• Cut back on discretionary foods such as chips, biscuits, fatty meats, pastries, cakes, fast foods and confectionery, and only eat them in small amounts.
• Eat regular meals and limit unhealthy snacking.
• Make half your plate vegetables at lunch and dinner.
• Use a smaller plate to help reduce your portion sizes.
• Swap sugary drinks and alcohol for water.
• Fit in regular physical activity, such as walking to the shops, taking the stairs and walking with a friend.
• Get moving every day and spend less time sitting.
• Set a realistic weight goal to help you feel motivated and active.

Starting a healthy eating and exercise plan together with your partner can increase your chance of getting pregnant and having a healthy baby.

Key points#

• Being overweight, obese or underweight can affect a woman’s fertility, and obesity can lower fertility in men.
• Even a small weight loss can improve fertility and pregnancy health.
• Obesity is linked with pregnancy complications such as miscarriage, high blood pressure, gestational diabetes and stillbirth.
• A healthy weight is also important when using IVF and other assisted reproductive technologies.
• If you are already pregnant and want to lose weight, talk to your doctor or a dietitian first.

Where to get help#