Why Aussie baby birthweight-charts needs revising

Photo: Getty Images/iStockphoto
Photo: Getty Images/iStockphoto 

Australian baby birth-weight charts need revising as they underdiagnose babies who are small for their gestational age. That's according to new research published this week in the Medical Journal of Australia, with experts saying their newly developed centiles could facilitate more accurate diagnosis of small for gestational age babies in Australia.

During pregnancy, baby birth-weight charts are used to asses babies' growth and to screen for small and large bubs who might be at risk of antenatal complications, complications during delivery or issues such as hypoglycaemia. 

According to the Australian Institute of Health and Welfare (AIHW), a baby may be small due to being born early, or be small for gestational age, which indicates a possible growth restriction within the uterus. "Poor fetal growth is associated with increased risk of stillbirth and with fetal distress during labour, and may make babies more likely to develop long-term health conditions later in life," they note.

Babies are defined as being small for gestational age if their birthweight is below the 10th percentile for their gestational age and sex, and large for gestational age if their birthweight is above the 90th percentile for their gestational age and sex, as determined by national percentiles.

Given the 10th centile is often used to define small for gestational age, who are at increased risk of perinatal complications. "The exact centile cut‐offs therefore have important implications for diagnosis and management during the ante‐ and postnatal periods," the authors write. 

Lead by Dr Farmey Joseph, of the Royal Prince Alfred Hospital, the team analysed data from the AIHW's National Perinatal Data Collection. This comprised of all liveborn Australian singleton infants delivered during 2004 - 2013 - with one exception. Babies who were born because of obstetric intervention were removed from the data.

"Removing the intervention group from birthweight-based growth curves excludes a significant source of bias and leads to more accurate diagnosis of pre-term small for gestation age, better identifying infants at risk of stillbirth and neonatal problems," the authors write.

"By restricting our analysis to spontaneous births, our charts more closely reflect normal growth and delivery trajectories by excluding the impact on growth curves for pre-term fetuses of early delivery by intervention of small for gestational age babies."

When they looked at the findings, the team noted that only three per cent of spontaneous births of less than 34 weeks' gestation fell beneath the previous 10th centile for birthweight.

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So what does this mean?

Put simply, a substantial proportion of small for gestation age pre-term babies are not recognised as such in Australia. According to the authors, using the new charts may mean that some babies previously classified as "normal" will now be identified as small for gestational age, "and may benefit from additional surveillance".

"Our new birthweight charts may facilitate more accurate diagnosis of small for gestational age babies and improve obstetric and neonatal care," the authors write.

The most recent data from the AIHW notes that babies were more likely to be small for gestational age if they were:

  • Indigenous
  • born to Indigenous mothers
  • born to mothers who live in Very remote areas
  • born to underweight mothers
  • born to teenage mothers (aged under 20)
  • born to mothers who smoked
  • born to mothers who lived in the lowest socioeconomic areas.