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Low birth weight in babies

Mum holding her newborn baby in hospital
Photo credit: iStock.com / manonallard
There are many different reasons why a baby may be born with low a birth weight. And how well a small baby grows after birth can depend on what caused their low birth weight in the first place. It could be to do with a mum's health before or during pregnancy, or it could be genetic, as small babies can run in families. But often there's just no reason for a baby to have a lower than average birth weight. Learn more about the causes of low birth weight, and how to help your baby thrive.

What does low birth weight mean?

You'll hear this term if your baby weighs less than 2.5kg (5.5lb) when they're born. About seven in 100 babies born in the UK have a low birth weight (NHS Digital 2020, OECD.Stat 2022).

If your baby weighs less than 1.5kg, midwives, doctors and nurses describe them as having a very, or extremely, low birth weight (Harding 2018). This can happen to babies who are born prematurely.

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If your baby was premature, their situation is different from babies who are small but are born between 37 weeks and 42 weeks (full-term).

You can read more in our premature baby section.

What causes low birth weight at full term?

There are many possible causes of low birth weight. They range through whether it runs in your family, the type of maternity care you have, health problems before and during pregnancy, and how many babies you're carrying.

Also, what's lower than average for all babies may not be the same for your family. Some babies are born small because people in their family tend to have a smaller stature compared with the average. And if parents were small themselves at birth, they're more likely to have a baby with a low birth weight (RCOG 2014, Mandy 2022).

Newborn twins tend to be smaller than average (Townsend and Khalil 2018). Twins grow more slowly than single babies in the final weeks of pregnancy (Balest 2021).

Low birth weight is more common in babies born to parents of Asian, African, and African Caribbean heritage, than babies who are born to White parents (Datta-Nemdharry et al 2012, RCOG 2014, NHS Digital 2020). This may be for many reasons, including the fact that women from minority ethnic groups may not always receive the healthcare they need during pregnancy (GOV.UK 2022, Jardine et al 2021).

Some pregnancy conditions can affect how unborn babies grow, especially conditions that affect the placenta, such as pre-eclampsia or high blood pressure (RCOG 2014, Mandy 2021).

Smoking during pregnancy, or taking illegal drugs, in particular cocaine, can also affect the placenta and cause low birth weight (RCOG 2014, NIDA 2016, Everson et al 2021).

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An unborn baby may have picked up an infection in the womb, such as cytomegalovirus (CMV), or rubella, which has affected their growth (Balest 2021, Mandy 2022).

Sometimes, a mum's mental health can affect how well an unborn baby grows (Barkmann et al 2017). It can be hard to take care of yourself when you're struggling with depression (Szegda et al 2014, Mind 2019).

Do tell your midwife or health visitor if you experienced depression while you were pregnant. Depression during pregnancy may make postnatal depression (PND) more likely. PND can start any time in your baby's first year, so ask for help whenever you need it (NHS 2022, NICE 2022).

Despite all these different causes for low birth weight, it may not be possible to work out why your baby was born small. In four out of 10 births, there’s no clear cause (Mandy 2022).

Read more about slow growth and low birth weight during pregnancy.

How could having a low birth weight affect my baby?

It's different for every baby. Your baby is likely to thrive after birth if their growth was only affected later in your pregnancy. Your baby may be a normal size but may just be a little thinner than expected, and with slightly looser skin. Your baby's head may be big in proportion to their body. That's because they need to build up muscle and fat (Balest 2021, Mandy 2022).

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Babies are born programmed to put on weight, so your baby should catch up quickly once they start feeding (Balest 2021, Mandy 2022). Breastfeeding is best, as breastmilk contains everything your baby needs to thrive (Mandy 2022).

Your first breastmilk is a concentrated, protein-packed substance called colostrum. Colostrum fills your baby's tummy, boosts their immune system, and even has a compound that programmes your baby to grow (Kim and Yi 2020).

At first, your baby may need medical help to get them on the right track. The following can affect term babies born with a low birth weight:

  • Breathing problems because your baby has breathed in some of their poo during labour. This is called meconium aspiration. It can happen when a baby's first poo, a black, dense substance called meconium, gets into the amniotic fluid. If your baby then takes big, deep breaths during or after birth because they're short of oxygen, the meconium can get into their lungs.
  • Low blood glucose (hypoglycaemia) because your baby may not have received enough sugar while in your womb. This means they haven’t had a chance to build up their sugar stores.
  • Too many red blood cells (polycythaemia). If your baby wasn’t getting enough oxygen while in the womb, their body will compensate by making more red blood cells. Too many cells makes your baby's blood flow more slowly and they may be a bit lethargic after birth.
  • Difficulty keeping warm because your baby hasn't laid down a layer of fat.
    (Balest 2021)

If your baby's weight has been lower than expected throughout your pregnancy, they may need more help, for longer, to catch up. That's because your baby will have grown more slowly overall during the months of your pregnancy, rather than just not filling out in the last few weeks (Balest 2021, Mandy 2022).

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In this case, your baby's slow growth rate may have been caused by an infection or illness that affected the placenta, or by an inherited condition (Balest 2021, Mandy 2022).

If your baby was born early, they may need to go to the neonatal intensive care unit (NICU) for treatment and monitoring (Mandy 2022). If your baby was full term, they can be cared for in the special care baby unit, or with you on the postnatal ward.

It'll help the medical team to give your baby the best treatment if they understand what's caused their low birth weight (Mandy 2022). They may:

  • Examine the placenta for signs of infection and to look for vessels that have been damaged by blockages or tears.
  • Look at your maternity notes and talk to you about your health and how your pregnancy went. Your medical team may ask to do blood and urine tests to find out if your baby picked up an infection in the womb.
  • Ask about your family history, to see if there are any inherited conditions that may have affected your baby's growth.
    (Balest 2021, Mandy 2022)

Will having a low birth weight affect my baby's development?

It's impossible to say. It's hard for researchers to measure children's development over a long period (Mandy 2022), as so many factors play a part, from having a healthy diet to being nurtured in a loving family.

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But many babies catch up on their growth charts in a matter of months (Mandy 2022).

Other babies may be affected in the longer term because something slowed their growth in the womb (Balest 2021, Mandy 2022). There are many different, complicated reasons for why this can happen, and every baby will develop at their own pace.

Some babies who were small because they were very poorly may take longer to reach milestones involving physical coordination and strength, and thinking skills (Balest 2021, Mandy 2022).

While your baby is young, there's plenty you can do to help them grow. It's best if your baby puts on weight steadily, rather than quickly. Breastfeeding can help with this (Elliot 2022, Mandy 2022). As well as special growth factors, breast milk also contains appetite regulators to ensure your baby takes in the right amount of milk at each feed (Kim and Yi 2020, Shah et al 2021).

Formula can help babies put on weight fast. However, there's a chance that fast weight gain may be unhealthier for your baby, because it can cause health problems later in life, such as diabetes and high blood pressure (Elliot 2022, Mandy 2022). There's no need to give your baby a high-calorie formula, unless your doctor advises you to (Elliot 2022, Hanson and Gluckman 2014).

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There is plenty more that you can do to help your child to stay healthy as they grow up.

Make sure your baby has all their growth and development checks (Mandy 2022) as well as all their vaccinations.

As your child gets older, keep an eye on their weight gain (Martin et al 2017, Visentin et al 2014). It's natural to want your child to gain weight if they started life as a small baby. But steady weight gain, that fits with the healthy range in their personal child health record (PCHR), or red book, is best.

A healthy, well-balanced diet can help your baby to grow steadily.
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Will my next baby have a low birth weight too?

There's a chance your next baby may have a low birth weight. It does run in families (RCOG 2013, Bhamidipaty-Pelosi et al 2021).

The midwives caring for you in your next pregnancy should offer you extra care and monitoring. If you had pre-eclampsia last time, your obstetrician may recommend you take daily aspirin (Hastie et al 2022, RCOG 2013). They will help you weigh up the pros and cons of taking medication while you're expecting a baby.

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You may also be offered extra growth scans in later pregnancy, which will include a Doppler scan to check the blood flow from the placenta to your baby (RCOG 2013).

Where can I get more help for my baby and me?

Talk to your midwife, health visitor or doctor about any problems or worries you have about your baby.

The charity, BlissOpens a new window, supports parents whose babies are born with a low birth weight.

For breastfeeding support:


Read about when your baby may have growth spurts.
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BabyCentre's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organisations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies.

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Chess Thomas

Chess Thomas is a freelance health writer and former research editor at BabyCentre.

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