- Slow weight gain, previously known as ‘failure to thrive’ or ‘faltering growth’, is when your baby does not gain weight or grow as quickly as expected.
- Development, brain growth and general health can all be affected when your baby’s weight and size do not increase over time.
- If a baby is consistently losing weight and not growing at the expected rate for their age and sex, they may be diagnosed as having slow weight gain.
- Babies may experience slow weight gain because of medical or environmental causes, or a combination of both.
- If your baby is diagnosed with slow weight gain, your doctor can advise you on the best way to treat it and monitor your baby’s growth.
What is slow weight gain?
Slow weight gain, also known as ‘failure to thrive’ or ‘faltering growth’, is when a baby does not gain weight or grow as quickly as expected.
Although every baby is unique, there are some expectations around the amount of weight they need to gain to grow in a normal healthy way. Development, brain growth and general health can all be affected when a baby’s weight and size do not increase over time.
All babies have periods when they gain or lose weight. Most babies lose weight after they are born — on average, up to 10% of their initial birth weight in their first week. However, the majority then regain their birth weight by 2 weeks of age.
How is slow weight gain diagnosed?
Babies who are diagnosed with slow weight gain weigh persistently below the third percentile of weight for their age, and/or have crossed 2 and more major percentile lines over time.
Babies diagnosed with slow weight gain weigh below the third percentile of weight for their age, and would be considered underweight.
Read more on Understanding baby growth charts.
If a baby is consistently losing weight and not growing at the expected rate for their age and sex, they may be diagnosed as having slow weight gain.
What causes slow weight gain?
Physical or medical causes
There are a few reasons why slow weight gain can happen. Sometimes there’s a physical or medical reason which causes your baby not to absorb or process nutrients. This means they are unable to use the food (milk or solids) they’re eating to grow.
Common medical conditions causing slow weight gain include:
- allergies to food and milk
- gastro oesophageal reflux disease (GORD)
- conditions that cause vomiting
- conditions that demand more energy that the baby takes in — cardiac (heart) problems, respiratory issues, infections or hormone disorders
Environmental factors can also cause slow weight gain. The most common reason is that your baby isn’t being fed the right amount of food to help them grow.
Some common environmental causes for slow weight gain include:
- the baby is not offered regular, nutritious milk and/or food
- family homelessness and/or poverty
- parental health issues including disability, chronic illness or mental health issues that may affect their ability to provide adequate food
- parents missing their baby’s cues for hunger
Combination of causes
Sometimes slow weight gain is caused by a combination of medical and environmental factors. Babies can refuse to feed or become fussy feeders because of an undiagnosed and untreated medical condition. Babies can also continue to be fussy feeders after they have recovered from being sick.
What if there’s no obvious cause for slow weight gain?
It’s not always clear what causes a baby to have slow weight gain. Sometimes, no specific cause is found. This makes it difficult for parents, who may feel they’re doing all they can to feed their baby regularly and support them to grow.
Slow weight gain doesn’t always mean something is wrong. Some babies may be smaller because of genetics. Parents who are shorter and smaller than average are more likely to have children with the same characteristics. Other babies may have a different growth pattern.
If the cause of slow weight gain isn’t clear, your doctor or maternal and child health nurse will continue to closely monitor your child’s growth and development.
It’s not uncommon for parents to feel stressed and anxious about their baby’s growth, especially when their baby is diagnosed with slow weight gain. If your baby is growing slower than expected, and this is causing you to feel stressed or anxious, be sure to let your doctor or midwife know. They will be able to refer you to support groups or professionals to help you cope.
How do I measure my baby’s growth?
It’s relatively easy to measure how your baby is growing. All babies need regular health checks with a health professional, and these will help to identify if a baby’s growth is slow. Certain markers used such as your baby’s weight, head circumference and length are all signs of growth.
On average, most babies will:
- double their birth weight by around 4 months of age, and triple it by 13 to 15 months (depending on your baby’s sex)
- increase their birth length 1.5 times in the first 12 months
- increase their birth head circumference by around 11 cm in 12 months
What happens if my baby has slow weight gain?
Your doctor or midwife may refer your baby to a paediatrician (specialist doctor for children). Sometimes, for babies up to 6 months, more frequent breast or formula feeds are enough to provide extra kilojoules (energy) to support their growth. For babies over 6 months, increased solids, can help with weight gain. If slow weight gain is not related to how your baby is feeding or an existing disease, your doctor can investigate further.
Resources and support
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.
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Last reviewed: June 2023