Why your Weight Matters During Pregnancy

Weight is a very sensitive subject for some women. However, because of the great benefit to you and your baby, it is recommended that you should try to reach a healthy weight before you become pregnant. By reaching a healthy weight, you are protecting your health and your baby’s wellbeing.  Women who are underweight or overweight have a higher chance of problems in pregnancy. Doctors and midwives often feel uncomfortable bringing up the issue of weight.  They are concerned you may feel judged. If you are not a healthy weight, it is important you to talk about this issue with your doctor or midwife so that you can try to reduce the increased chance of complications.

What is BMI?

Your body mass index (BMI) is a comparison of your weight to your height. Your BMI will be calculated at your first antenatal appointment. A healthy BMI is above 18.5 and below 25.  Many women are unaware of the how much weight they should put on during pregnancy and some gain more than is ideal. There is no need for you to ‘eat for two’, as was previously thought. The table shows the recommended range of weight gain in pregnancy by BMI.  

Your doctor or midwife will be able to provide general nutritional information to help you achieve the optimal weight gain during your pregnancy.

What are the problems associated with a low BMI during pregnancy?

Women who are very underweight have an increased chance of miscarriage and their babies may have an increased chance of prematurity, low birth weight and nutritional problems. It is essential that you and your baby receive the vitamins and minerals you require. Most of these can be gained through good nutrition and a healthy diet.

What are the problems associated with a high BMI during pregnancy?

Most pregnant women who have a high BMI can expect to enjoy a healthy pregnancy. However, having a raised BMI increases the chance of complications for both you and your baby. The higher your BMI, the higher the risks.  As your BMI increases, so does the likelihood of one or more of the following problems occurring:

  • A blood clot in the leg (deep vein thrombosis) or in your lungs (pulmonary embolism)
  • Gestational diabetes, a form of diabetes that develops during pregnancy
  • High blood pressure and pre-eclampsia
  • Difficulties with some procedures – having too much body fat can make it difficult to monitor your baby’s heartbeat, view certain problems with the baby’s anatomy on an ultrasound scan and to give you an epidural.

The problems for your baby associated with a high BMI include:

  • Problems with the development of the baby’s brain and spine (neural tube defects)
  • Higher rate of miscarriage
  • A birth weight greater than 4kg
  • Admission to a special care nursery
  • Higher rate of stillbirth
  • Increased chance of obesity and diabetes later in life

What are the problems associated with a high BMI during labour and birth?

There is an increased risk of complications during labour and birth, particularly if you have a BMI above 40. If your BMI is above 40 you may need to have your baby at a hospital with the appropriate facilities and experienced clinicians to provide the specialised care that meets your needs.  Some of the problems include:

  • Your baby being born prematurely (before 37 weeks)
  • Difficulty monitoring the baby’s heartbeat
  • Anaesthetic complications
  • Greater likelihood of requiring an emergency caesarean section
  • Shoulder dystocia, which is when the baby’s head is born, but the shoulders do not come out. The doctor or midwife will take steps to help the shoulders to be born, and this can be frightening
  • Heavy bleeding after birth (postpartum haemorrhage)

Because of these possible complications, you should have a discussion with your obstetrician or midwife about the safest way and place to give birth.

How can the chance of having these problems be reduced?

Despite having a high BMI, you can still have a healthy pregnancy. It takes careful management of your weight, attention to diet and exercise, regular antenatal care to monitor for complications, and special considerations for your labour and birth. By working together with your healthcare team, the chance of having problems can be reduced for both you and your baby. Healthy diet and an
active lifestyle are important. A healthy diet will provide benefits both during your pregnancy and after the birth.  Your doctor or midwife may refer you to a dietitian to help you plan a healthy diet or recommend specific supplements.  For women with a BMI over 40, your doctor may advise a more limited weight gain than 5–9kg. Further information about exercise during pregnancy can be found on the RANZCOG website under patient information.

After pregnancy

Once you are home with your baby, stick to your healthy eating and exercise habits to help you reach a normal weight. Highly restrictive diets are not recommended after the birth of your baby, particularly if you are breastfeeding.  Breastfeeding is recommended for the first year of a baby’s life. Not only is breastfeeding the best way to feed your baby, it may also help with weight loss. Overall, women who breastfeed their babies for at least the first few months tend to lose the weight they gained during pregnancy faster than women who do not breastfeed.  Getting to a healthy weight after pregnancy reduces your risks in future pregnancies as well as improving your longterm health.

DISCLAIMER: This information is intended to be used as a guide of general nature, having regard to general circumstances. The information presented should not be relied on as a substitute for medical advice, independent judgement or proper assessment by a doctor, with consideration of the particular circumstances of each case and individual needs. This information reflects information available at the time of its preparation, but its currency should be determined having regard to other available information. RANZCOG disclaims all liability to users of the information provided.