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Exercise During Pregnancy for Smaller Baby

Moderate Exercise During Pregnancy Lowers Baby's Birth Weight, May Reduce Baby's Risk of Obesity

Written by Kathleen Doheny

April 5, 2010 -- Women who exercise moderately during pregnancy give birth to somewhat smaller babies, which may reduce the infants' obesity risk later, according to a new study.

The average birth weight of babies born to exercising mothers was lower but still very healthy.

The average birth weight of babies born to mothers who exercised was 7.5 pounds, compared to 7.8 pounds for mothers who did not exercise, says a team of researchers from New Zealand and the U.S. Babies born weighing 8.8 pounds or more are defined as high birth weight.

At the two-week checkup, the babies of exercising moms averaged 8.1 pounds; the babies of sedentary moms averaged 8.6 pounds.

"We would suggest this study support the recommendations of at least 30 minutes of moderate exercise daily and probably more," says study co-author Paul Hofman, MD, a researcher at the University of Auckland in New Zealand. The study is published in the Journal of Clinical Endocrinology & Metabolism.

The new research follows a recent study finding that three out of four pregnant women in the U.S. do not get enough exercise.

Exercise During Pregnancy: The Study

Hofman and his colleagues assigned 84 women pregnant with their first child to an exercise group or a control group. Women in the exercise group rode stationary bikes at home at a moderate intensity for 40 minutes, 5 times a week maximum, beginning at 20 weeks into the pregnancy and continuing until about week 36.

Women in the control group were instructed to continue their normal daily activities during the same time periods.

Women in both groups had, on average, a healthy body weight before pregnancy and were similar in other regards such as age and ethnicity.

Exercise During Pregnancy: Results

The babies born to the exercisers had a lower body weight and a lower body mass index or BMI.

There were no differences in the length of the babies, on average, between exercisers and non-exercisers. The exercise didn't affect the length of the pregnancy, either, or the mothers’ weight.

Exactly why the exercising mothers produced smaller babies isn't known, says study co-author Chris Baldi, PhD, an assistant professor of biological sciences at Northern Arizona University, Flagstaff.

Researchers initially believed the exercising mothers would become less insulin-resistant. Typically, Baldi says, a woman becomes more insulin-resistant during pregnancy, a normal occurrence within certain boundaries. "It has a lot to do with hormonal changes," he says. If a woman becomes too insulin-resistant, gestational diabetes can occur.

"It did not come out as we suspected," Baldi tells WebMD, with no substantial differences found in insulin resistance between the two groups.

The difference, says Baldi and Hofman, may be because of other factors, such as differences found in certain growth factors. Another possible explanation, Hofman says, is slightly reduced blood flow that occurs to the placenta during exercise and for several hours afterward.

Exercise During Pregnancy: Other Input

The finding of lower birth weights of infants of exercising mothers is not surprising to Linda May, PhD, an associate professor of anatomy at the Kansas City University of Medicine and Biosciences in Kansas City, Mo., who has also researched the topic.

The finding that there was no change in insulin sensitivity between groups was surprising, she tells WebMD.

Her research has found that during pregnancy, more exercise [within recommended amounts for an individual] yields more benefits.

More study is needed on high-risk pregnant women, Hofman and May say. The women in the study, exercisers and non-exercisers, were not high risk.

For most healthy pregnant women, Hofman says, the research seems to support exercising at least 30 minutes a day moderately, and probably more. Some studies, he says, have demonstrated up to 60 minutes daily is generally safe.

Having a small baby | Pregnancy Birth and Baby

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Some babies may be smaller than others during pregnancy. However, what’s important is that your baby grows at a healthy rate in your womb, regardless of their size.

Here’s what you need to know about your pregnancy if you have been told by your doctor or midwife that your baby is small or growing slower than expected. Your doctor or midwife may refer to a slower growth as ‘small for dates’, ‘small for gestation’ or ‘fetal growth restriction’.

Why is my baby small?

Most babies who are smaller than expected are healthy. In some cases, babies are small and appropriately grown, given the size of their parents and their ethnicity.

However, up to 10% of pregnancies will be affected by ‘fetal growth restriction'. This is a more serious complication that means your pregnancy must be closely monitored. In many instances, babies with fetal growth restriction are small because the placenta hasn’t developed well enough to keep up with the baby’s growing needs for nutrients and oxygen, although a cause may not always be found.

Risk factors for having a baby with fetal growth restriction include:

  • previously having a small baby, pre-eclampsia or a stillbirth
  • an infection that passes across the placenta, such as cytomegalovirus (CMV)
  • heavy bleeding early in pregnancy
  • having a chronic health condition, such as high blood pressure, kidney disease or diabetes
  • severe anaemia or undernutrition
  • smoking, drinking alcohol or using illegal drugs
  • a genetic or chromosomal condition in the baby
  • having a multiple pregnancy (twins, triplets or more)

How is a baby's size measured?

During routine antenatal check-ups, your doctor or midwife will use a tape measure to estimate the growth and size of your baby by measuring your abdomen, from your pubic bone to the top of your uterus (or fundus). This is known as the ‘symphyseal fundal height measurement’ (SFH) and gives some indication about a baby’s growth during pregnancy.

If you have risk factors for fetal growth restriction, or if your baby has been diagnosed with this, your baby’s growth may be monitored more closely by ultrasound, instead of using a tape measure.

Every pregnancy is unique. Your doctor or midwife will track your baby’s growth at each check-up. They will talk with you about next steps if there are signs that your baby’s growth has slowed.

Should I be concerned if my baby is small?

Most small babies will be healthy. However, if your baby has fetal growth restriction, this increases the risk of complications and, sadly, also the risk of stillbirth. Your doctor and midwife will monitor you and your baby’s health closely to reduce the risk of this happening.

You can monitor your baby’s health by attending your scheduled antenatal visits and keeping a close eye on your baby’s movements. If you are concerned about your baby’s movements at any stage, call your doctor or midwife immediately.

Your doctor or midwife may also talk to you about the best time for your baby to be born. Sometimes it may be necessary for your baby to be born early, before your due date. Every situation will be different; your doctor or midwife will give you personalised advice each step of the way.

Generally a baby with fetal growth restriction can still be born via normal vaginal delivery, but you and your baby may need extra care both during labour and after the birth. This means it’s best to give birth where you can access specialist medical services.

Talk to your doctor or midwife about the best place for you to give birth.

Can I do anything to reduce the risk of having a small baby?

Often there is nothing you can do to prevent a baby being small. But looking after yourself during pregnancy is important to help reduce the risk of having a baby with fetal growth restriction. You should:

  • quit smoking (if you currently smoke)
  • eat a balanced, healthy diet
  • keep a healthy weight during pregnancy
  • avoid alcohol and illegal drugs

What to expect if your baby is born with low birth weight?

If your baby weighs less than 2.5kg at birth, their head may appear to be a lot bigger than the rest of their body. They may look thin with little body fat.

Babies of low birth weight or those who are born prematurely, may need to be admitted to the neonatal intensive care unit (NICU) or special care nursery (SCN).

Some of the challenges faced by babies with low birth weight can include:

  • breathing or heart problems
  • low oxygen levels at birth
  • difficulty maintaining body temperature
  • difficulty feeding and gaining weight
  • infection
  • bleeding on the brain (called ‘intraventricular haemorrhage’)
  • problems with their eyes and vision
  • problems with their intestines

These conditions are more likely to occur in premature babies and those with fetal growth restriction.

If there are no other complications, low birth weight babies usually 'catch up' in their physical growth. In later life, however, people who were born smaller than average are more likely to develop diabetes, obesity, heart problems and high blood pressure.

Questions you might want to ask your doctor

Here are some questions you might want to ask your midwife or doctor:

  • What does having a small baby mean for me and the health of my baby?
  • Will I need more regular check-ups during pregnancy? If so, what does this include?
  • How will I know if my baby is well?
  • Will having a small baby affect how and where I can give birth?
  • Does having a small baby affect my next pregnancy?

Who can I talk to for advice and support?

Always talk to your doctor or midwife first if you have any concerns about your pregnancy, your own health or the health of your baby.

Speak to a maternal child health nurse

Call Pregnancy, Birth and Baby to speak to a maternal child health nurse on 1800 882 436 or video call. Available 7am to midnight (AET), 7 days a week.

Sources:
Stillbirth Centre of Research Excellence (Your baby's growth matters), Department of Health (Pregnancy care 2020 edition), The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (Perinatal post-mortem examination)

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: April 2022


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Related pages

  • Neonatal intensive care unit (NICU)
  • Special care nursery (SCN)
  • How your baby gains weight
  • Premature baby
  • Having a large baby

Need more information?

Low-birth weight babies - MyDr.
com.au

Babies are considered of low birth weight if they weigh less than 2500 g at birth. Low birth weight is associated with an increased risk of disabilities.