Gestational Age-Specific Stillbirth Rates: Are We Using the Right Denominator?

Globally, a stillbirth occurs every 17 seconds totalling almost 2 million every year. The global standard for reporting stillbirths is the stillbirth rate. While a critical and accessible summary statistic, it masks gestational age-specific trends.

This study aimed to summarise and critique stillbirth trends and document gestational age-specific rates and risk calculations according to three published methodologies: gestational age-specific stillbirth rate (GS-SBR), foetus-at-risk (FAR) stillbirth rate, and continuity corrected foetus-at-risk (ccFAR) stillbirth rate.

We conducted a cross-sectional analysis of all births from the National Perinatal Data Collection in Australia (1998-2018). Stillbirth was defined as the birth of a baby with no sign of life from 20 weeks of gestation or weighing 400 grams at birth. Three statistical measures were applied reflecting alternate denominators: the GS-SBR measure used the number of births in a given week, the FAR used the number of foetuses in utero at the start of a week, and the ccFAR used the number of foetuses at the midpoint of the week.

In Australia from 1998 to 2018, there were 39,576 stillbirths among 5.9 million births, an overall stillbirth rate of 6.7 per 1000 births. For each week of gestation (20-41+), the average GS-SBR varied from 1.0 to 790.0 stillbirths per 1000 births; FAR varied from 0.1 to 1.2 stillbirths per 1000 FAR; and ccFAR varied from 0.1 to 2.4 stillbirths per 1000 ongoing pregnancies. The three stillbirth rate calculations showed distinct trends during pregnancy, and only FAR and ccFAR reflected increased stillbirth risk as gestation advances.

When calculating gestational-age-specific stillbirth rates, the FAR or ccFAR approaches provide the most accurate representation of the probability of stillbirth throughout pregnancy and better enable clinicians to act on risk.