Comparing regional infant death rates: the influence of preterm births <24 weeks of gestation

OBJECTIVE To investigate regional variation in the registration of preterm births <24 weeks of gestation and the impact on infant death rates for English Primary Care Trusts (PCTs). DESIGN Cohort study. SETTING England. PARTICIPANTS All registered births (1 January 2005-31 December 2008) by gestational age and PCT (147 trusts) linked to infant deaths (up to 1 year of life). Late-fetal deaths at 22 and 23 weeks gestation (1 January 2005-31 December 2006). MAIN OUTCOME MEASURES Extremely preterm (<24 weeks) birth rate per 1000 live births and percentage of births registered as live born by PCT. Infant death rate and rank of mortality for (1) all live births and (2) live births over 24 weeks gestation by PCT. RESULTS Wide between-PCT variation existed in extremely preterm birth (<24 weeks) rates (per 1000 births) (90% central range (0.31, 1.91)) and percentages of births <24 weeks of gestation registered as live born (median 52.6%, 90% central range (26.3%, 79.5%)). Consequently, the percentage of infant deaths arising from these births varied (90% central range (6.7%, 31.9%)). Excluding births <24 weeks, led to significant changes in infant mortality rankings of PCTs, with a median worsening of 12 places for PCTs with low rates of live born preterm births <24 weeks of gestation compared with a median improvement of four ranks for those with higher live birth registration rates. CONCLUSIONS Infant death rates in PCTs in England are influenced by variation in the registration of births where viability is uncertain. It is vital that this variation is minimised before infant mortality is used as indicator for monitoring health and performance and targeting interventions.