Gestational age, birthweight and plurality

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

Of all perinatal deaths in 2015 and 2016:

  • Rates were higher for babies born with a birthweight less than 1,000 grams and for babies born at less than 28 weeks gestation.
  • Nearly half occurred from 20 to 23 completed weeks gestation (49.2%).
  • Rates decreased rapidly from 28 weeks gestation and were particularly low over 34 weeks gestation.
  • Rates were highest among babies who were small for gestational age (21.9 per 1,000 births).
  • The rate for twins was almost 4 times that of singletons (32.6 deaths per 1,000 births), and for higher order multiples (triplets or higher pluralities) was 10 times that of singletons (84.6 deaths per 1,000 births).

Gestational age trend

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

  • Stillbirth rates have decreased for babies born from 32 weeks gestation.
  • Neonatal mortality rates have seen the most marked decrease from 23 weeks gestation.

Perinatal mortality risk

The gestational age-specific risk of perinatal mortality is the chance of a perinatal death occurring within a specified gestation interval (refer to the Technical notes—Methods for more information on the risk calculation).

The risk of a baby dying in the perinatal period was highest:

  • after 41 completed weeks gestation
  • before 24 weeks gestation.