During a healthy pregnancy, women gain weight as the baby grows. This is normal and necessary.

But recent research indicates that excessive weight gain during pregnancy increases health risks for mothers and their children, according to The Daily Mail.

Yet half of mothers-to-be gain too much weight, according to research involving half a million pregnancies published in JAMA last year.

Even women who started pregnancy at a healthy weight (those with a BMI of 18.5 to 24.9) typically piled on too many kilos.

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So how do you get the right balance? You do not need to eat any extra food to support the baby's growth – however, experts warn the 'eating for two' mantra is dangerous.

Writing for The Conversation, Clare Collins, Professor in nutrition and dietetics from the University of Newcastle, Dr Jenna Hollis, conjoint lecturer at the University of Newcastle and Siân Robinson, Professor of nutritional epidemiology at the University of Southampton, answer the question and give their tips for gaining weight healthily.

They carried out a study that found excessive weight gain was more common among UK women who were having their first baby.

The experts stress that it's important for mothers-to-be to calculate their recommended weight gain in pregnancy based on their body weight and body mass index (BMI) prior to them becoming pregnant.

"Most weight gain occurs from week 13," they explain.

"For some women, body weight will not change too much during the first trimester of pregnancy, particularly for women who have had morning (noon and night) sickness."

So why is it important?

"Excess weight gain has been linked to a higher risk of developing diabetes in pregnancy, high blood pressure, and complications during birth," said the trio.

"It can also affect the health of the baby in both the short-term and in the future."

Indeed, children with obese mothers are 36 per cent more likely be autistic and a 62 per cent more likely to have attention deficit hyperactivity disorder (ADHD), according to findings from the Virginia Commonwealth University last November.

Obese pregnant women also have a higher risk of birth complications from delivering an overgrown baby, found a study by the National Institutes of Health during the same month.

And children of mothers who gained too much weight were then at a higher risk of becoming obese as a child or adult, found a study by researchers from the School of Population Health at the University of Queensland, Brisbane, which was published in 2014 in the journal Obesity Reviews.

Excess pregnancy weight gain can also make it more challenging to lose weight after the baby is born, warns Professor Clare Collins and her co-authors.

"Our recent study showed that women who gained more weight than recommended retained, on average, an extra 4kg six months after their baby was born.

"Of concern is that this extra weight can still be retained decades after pregnancy. Not shifting those extra few kilos after pregnancy increases the chances of developing obesity in the future."

Warning not to 'diet' during pregnancy

Half of mothers-to-be are gaining too much weight, say researchers. Photo / Getty Images
Half of mothers-to-be are gaining too much weight, say researchers. Photo / Getty Images

However, they also warn that not gaining enough weight in pregnancy can also be a problem.

"Weight gain below the recommendations is associated with having a baby that is small for gestational age, or a preterm birth," they explained.

"It's important for women not to try and lose weight during pregnancy. Dieting or limiting food intake could mean the baby doesn't get enough nutrients needed for their development."

The NHS states that you don't need to go on a special diet, but it's important to eat a variety of different foods every day to get the right balance of nutrients that you and your baby need.

It says it is best to get vitamins and minerals from the foods you eat, but recommends taking a folic acid supplement as well.

FIVE TIPS FOR HEALTHY PREGNANCY WEIGHT GAIN

Professor Collins, Dr Jenna Hollis, and Professor Siân Robinson provide their advice for ensuring you eat the right amount when you're expecting.

1. Start a conversation with a doctor or midwife

While talking about weight during antenatal visits can make some pregnant women feel anxious, knowing how much weight is appropriate can help improve the pregnancy outcomes for women and their infants.

Having this conversation is important because a doctor or midwife can provide support. They can also refer pregnant women to a dietitian or other service, if needed.

2. Track weight gain from early in pregnancy

Monitoring weight in pregnancy can help keep weight gain 'on track'. Try a pregnancy weight tracker; such as this one for women with a pre-pregnancy BMI of less than 25, or this one for women with a pre-pregnancy BMI of more than 25. Start recording weight as early in pregnancy as possible.

Remember every pregnancy is different, and the amount of weight gained each week won't be identical. But it's a great way to keep check and see whether weight gain patterns are tracking above or below the recommendations.

3. Focus on healthy eating

It's a myth that you need to 'eat for two' during pregnancy. During the first trimester, dietary energy needs (measured in calories or kilojoules) are only slightly higher so the amount of food eaten should remain about the same.

But nutrient requirements increase, particularly for folate, iodine and iron, so women need to be mindful of the nutritional quality of food eaten.

While more food energy is needed during the second and third trimester, the amount of extra food is less than most people think. It would be the equivalent of a sandwich (such as egg, beef, hummus or cheese), or a yoghurt and banana.

Use the Eat-for-Health Calculator to calculate the recommended daily serves from the five food groups to give you an idea of what you should be eating during pregnancy.

4. Exercise regularly

Being active is important during pregnancy. The national recommendations advise the accumulation of 150 minutes of exercise each week.

Many exercises are safe during pregnancy, such as walking, swimming, stationary cycling and pregnancy-specific exercise classes. Doctors, midwives, exercise physiologists and physiotherapists can provide advice about the best options.

5. Enlist the support of a partner and family

Having a healthy lifestyle that includes eating healthily and being active is not just important for the mother and baby in pregnancy, but can also benefit other family members.

WEIGHT GAIN GUIDELINES

If you've having one baby and your pre-pregnancy BMI is:

Less than 18.5 – aim to gain between 12.5 and 18kg (28–40Ibs)
18.5 to 24.9 – aim to gain 11.5 to 16kg (25–35Ibs)
25.0 to 29.9 – aim to gain 7 to 11.5kg (15–25Ibs)
30 or more – aim to gain 5 to 9kg (11–20Ibs)

If you've having twins and your pre-pregnancy BMI is:

18.5-24.9 – aim to gain 17-25kg (37–54Ibs)
25-29.9 – aim to gain 14-23kg (31–50Ibs)
30 or more – aim to gain 11-19kg (25–42Ibs)

Source: Institute of Medicine (IOM)