Babies are considered of low birthweight if they weigh less than 2500 g at birth. Some low birthweight babies are healthy but others need special care. Low birthweight is associated with a risk of illness in infancy and a long term increased risk of developing some chronic diseases and a risk of poor mental development.
This risk becomes significant for those babies born weighing less than 1500 g. Doctors usually further classify these babies into:
- very low birthweight (VLBW) – if they weigh 1000–1500 g
- extremely low birth weight (ELBW) – if they weigh less than 1000 g.
Babies of low birthweight may be premature babies – babies who are born pre-term (before 37 weeks), but can also be babies who are born at term, but who are small. The very low birthweights and extremely low birthweights are seen in infants who are born prematurely.
What percentage of babies are low birthweight?
In Australia in 2017 nearly 7 per cent of babies born were of low birthweight. This equated to around 20,300 babies. In Aboriginal and Torres Strait Islander communities, the percentage of babies of low birthweight is much higher than the Australian average.
Is low birthweight the same as ‘small for gestational age’?
No, the two terms do not mean the same thing. Small for gestational age (SGA) is not the same as low birthweight. “Small for gestational age” (SGA) is the term given to fetuses/babies who have a weight below the 10th percentile for that gestational age and/or who have a birthweight below the 10th percentile.
A small for gestational age baby may have a low birthweight or it may not.
You could have a premature baby born at 35 weeks, and it may weigh less than 2500 g (low birthweight), but it may still have a weight that’s appropriate for its gestational age. It just didn’t get to put on weight for those last weeks before its full term due date.
Similarly, a baby born at term that is small for gestational age may not necessarily have a low birthweight.
Risk factors that increase the chance of having a low birthweight baby
Having your baby born too early (pre-term birth) is probably the most common reason why a baby is of low birthweight, but there are other risk factors that make you more likely to have a low birthweight baby, such as:
- being aged less than 16 or more than 40 when pregnant
- being underweight
- being overweight
- having high blood pressure / pre-eclampsia during pregnancy which often necessitates an early delivery
- smoking during pregnancy
- taking illicit drugs
- excessive alcohol consumption
- inadequate nutrition
- being an Indigenous mother
- having twins, triplets or other multiple pregnancy
- having diabetes
- having a complication with the placenta.
Is special care needed for low birthweight babies?
Many low birthweight babies are healthy, just a bit small. But some low birthweight babies may have breathing problems and difficulty staying warm. They may have more trouble feeding and gaining weight than normal weight babies. They may need to stay in a special care nursery or the neonatal intensive care unit (NICU).
Problems in low birthweight babies as a newborn
The following are some problems that may be associated with low birthweight in newborn babies.
Low birthweight babies may have low oxygen levels at birth. Their lungs may not be fully matured, which can lead to respiratory distress. They may need oxygen and help with breathing.
Jaundice happens when the liver is not working efficiently enough to clear the accumulation of bilirubin in the baby’s blood. A baby with jaundice may need treatment in hospital and may need extra feeding and help with feeding to get better.
Feeding problems and problems gaining weight are not uncommon in low-birth-weight infants. Premature babies may not have developed the ability to suck and so may need to be tube fed for a little while until they mature.
Babies that are born prematurely may not have fully developed immune systems and so may be more vulnerable to infections.
Problems affecting low birthweight babies in childhood
Low birthweight babies may be born pre-term or at term and represent a broad group, so there can be a wide spectrum of physical and developmental outcomes when they reach childhood.
Low birthweight babies are at significantly increased risk of having asthma in childhood.
Some studies have shown a link between the quality of motor development in the first year of life of low birthweight babies and their cognitive (thinking/intellectual) skills later on.
Children born with low birthweight have worse cardiorespiratory fitness than those born of heavier weight, according to a large study. Children born very prematurely generally have a lower exercise capacity than those who were born at full term.
Once low birthweight infants reach the toddler stage, growth tends to occur at a consistent and steady rate. But it’s important to remember that when their height and weight are plotted on standard growth charts, children born prematurely may be below the average for their age.
While severe functional problems that limit a child’s ability to perform everyday tasks such as dressing, toileting and feeding are not common, they do occur more often in children who were born extremely prematurely than in children who were born at full term.
In addition, many ELBW infants will go on to experience milder functional impairment, such as speech difficulties or trouble understanding simple instructions. As a result, they are more likely to need physiotherapy, speech therapy or occupational therapy. Regular reviews by appropriate health professionals will help in the early detection of any difficulties and help maximise your child’s functional and educational opportunities.
Will I know if I am carrying a low birthweight baby?
The size of your baby can be estimated during your routine antenatal check ups, but you won’t know its weight. Routine ultrasounds will also give an indication of the baby’s size, but can’t accurately predict its weight.
Last Reviewed: 05/11/2020
1. AIHW. Australia’s children. Last updated April 2020. https://www.aihw.gov.au/reports/children-youth/australias-children/contents/health/birthweight
2. Oudgenoeg-Paz O, Mulder H, Jongmans MJ, van der Ham IJM, Van der Stigchel S. The link between motor and cognitive development in children born preterm and/or with low birth weight: A review of current evidence. Neurosci Biobehav Rev. 2017 Sep;80:382-393. doi: 10.1016/j.neubiorev.2017.06.009. Epub 2017 Jun 19. PMID: 28642071. Format:
3. Xu XF, Li YJ, Sheng YJ, Liu JL, Tang LF, Chen ZM. Effect of low birth weight on childhood asthma: a meta-analysis. BMC Pediatr. 2014 Oct 23;14:275. doi: 10.1186/1471-2431-14-275. PMID: 25339063; PMCID: PMC4288645.
4. Ahlqvist VH, et al. Birth weight and cardiorespiratory fitness among young men born at term. The role of genetic and environmental factors. J Am Heart Assoc 2020; 9 (3). https://www.ahajournals.org/doi/10.1161/JAHA.119.014290