As your baby grows, you will gradually gain weight. Gaining less or more weight than is recommended can have health implications for you and your baby, such as too much weight gain increasing the risk of gestational diabetes. If you have concerns about how much weight you are gaining (or not) in your pregnancy, discuss this with your doctor or midwife.

What is normal weight gain in pregnancy?

How much weight you gain will depend on how much you weighed before your pregnancy.

To calculate how much you should gain, first work out your pre-pregnancy body mass index (BMI). The formula for calculating BMI is:

Your pre-pregnancy weight (in kilograms) divided by the square of your height (in metres).

So if you weighed 68kg and you’re 1.7m tall, your BMI calculation would be 68 / 1.7 x 1.7 = 23.5.

You can use the healthdirect BMI calculator to work out your pre-pregnancy BMI.

If your BMI was 18.5 to 24.9, you were in the healthy weight range before becoming pregnant, and ideally you should gain between 11.5kg and 16kg: 1 to 1.5kg in the first 3 months then 1.5 to 2kg each month until you give birth.

If you were above the healthy weight range, you should gain less. If you are below the healthy weight range, you should gain more.

Your weight gain can also be affected by:

• carrying twins
• having morning sickness

Talk to your doctor about what’s the best weight gain for you.

Why am I gaining weight?

Not only is your baby growing, but your body is also developing extra body tissue. You will put on weight because:

• your breasts grow larger
• your uterus grows bigger
• there is amniotic fluid around the baby
• the placenta grows larger
• your body creates extra blood and fluid

What are the problems with gaining too much weight?

Your weight gain will be monitored throughout your pregnancy. If you gain more than 16kg, you and your baby could be at greater risk of complications such as:

• gestational diabetes
• hypertension
• caesarean section
• having a large baby (macrosomia)
• stillbirth

Babies born to mothers who put on too much weight are more likely to develop overweight and obesity in later life, develop more health problems, and be born with heart disease (especially if you smoke as well).

Managing your weight gain

You can help put on the right amount of weight by:

• eating a healthy, nutritious diet that includes fresh fruit and vegetables, wholegrain breads and cereals, legumes, lean meat, fish and low-fat dairy products
• avoiding fatty and sugary foods and drinks
• getting some regular moderate exercise

Make sure you know which foods are safe to eat during your pregnancy.

How much more food should I eat?

It’s important to eat well when you’re pregnant to give your baby a healthy start. But you don’t have to ‘eat for 2’, as some well-meaning people may have suggested.

You’ll probably find you don’t need to consume too many extra kilojoules in the first 3 months. As your baby grows, an extra 1,400 to 1,900 kilojoules a day in the second and third trimesters is likely to provide a healthy weight gain. It’s best to add that extra kilojoules through healthy food. This includes fresh fruit and vegetables, wholegrain breads and cereals, legumes, lean meat, fish and low-fat dairy products.

You should check that your diet contains the nutrients that keep you healthy and that will give your baby a healthy start such as folic acid, iron, calcium, iodine and protein.

It’s important to avoid foods that are high in sugar and/or fat and that don’t provide any vitamins or minerals.

Keeping up your fluid intake is also important — it’s recommended you drink about 2L of water each day. Morning sickness can make you dehydrated, so talk to your doctor or health professional if you’re not retaining enough fluids.

How much should I exercise?

Unless your doctor advises otherwise, you can start or continue with regular exercise when you’re pregnant as long as you adjust your activity to suit your stage of pregnancy. About 30 minutes each day of walking, swimming or pregnancy exercise classes will help — but don’t do more than 20 minutes of fast physical activity at a time, to avoid overheating.

Walking, swimming, aqua aerobics and pregnancy exercise classes are good choices. They will help prevent you from putting on extra weight, reduce your risk of gestational diabetes, and make you fitter so you can cope with labour better.