Baby characteristics

Birthweight and gestational age are interrelated and birthweight is generally expressed in relation to gestational age using population percentiles (refer to the Technical notes—Methods for more information on percentiles).

Gestational age and birthweight

A baby may be small due to being pre-term (born early), or due to being small for gestational age (either due to genetic factors, or because it is the subject of a growth restriction within the uterus). Poor fetal growth is associated with increased risk of perinatal death and with fetal distress during labour, and may make babies more likely to develop long-term health conditions later in life.

Adjusting birthweight for gestational age allows for differences in a baby’s growth status and maturity to be taken into account when examining their health at birth.

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

In 2017:

  • Nearly 2 in 5 perinatal deaths (39.4%) occurred before 22 completed weeks gestation
  • Rates of perinatal death decreased rapidly from 28 weeks gestation and were lowest among babies born at term (37-41 weeks).

The highest rates of perinatal death were among:

  • Babies born at less than 27 weeks gestation
  • Babies born with a birthweight less than 2,500 grams
  • Babies who were small for gestational age (birthweight below the 10th percentile for their age and sex)
  • Multiple births.

Perinatal mortality rates by birthweight for gestational age, 2017

The vertical bar charts show that the rates of both stillbirth and neonatal death were highest in babies born small for gestational age, or those with a birthweight below the 10th percentile for their gestational age and sex. The highest rates were in babies whose birthweight for gestational age was below the 3rd percentile for their gestational age and sex. The stillbirth rate for this category was 50.0 deaths per 1,000 total births, and the neonatal death rate was 8.3 deaths per 1,000 live births.

Gestational age trend

Perinatal mortality rates have been gradually decreasing among particular gestational age groups, most notably among babies born at 27 weeks gestation or more.

  • Stillbirth rates have decreased for babies born at 32 weeks gestation or more.
  • Neonatal mortality rates have decreased for babies born at 23 weeks gestation or more.

Perinatal mortality rates, by gestational age groups, 1998-2017

The stacked continuous line graph shows that the perinatal mortality rate for babies born at 20-22 weeks gestation has increased from 984.8 deaths per 1,000 births in 1998 to 997.4 deaths per 1,000 births in 2017. This is the only gestational age group that has seen an increase in the perinatal mortality rate.