1. The rate and risk of mortality <28 days from birth was the highest in infants born with both preterm birth (PTB) and small for gestational age (SGA) status, followed by those born with PTB without SGA, then term birth (TB) with SGA (TB-SGA) and TB without SGA.
2. When stratifying SGA into 23-31 weeks’ gestational age (GA) and 32-36 weeks’ GA, the PTB-SGA 23-31 GA group had a higher rate and risk of mortality compared to the PTB-SGA 32-36 week group.
Evidence Rating: 1 (Excellent)
Study Rundown: Preterm birth (PTB) and small for gestational age (SGA) are factors that increase risk of neonatal mortality. The majority of infant deaths typically occur between birth and 27 days of life. Few studies have evaluated the association between mortality and preterm birth with small for gestational age (PTB-SGA) as a combined risk factor. In this retrospective cohort study, researchers compared both the rate and relative risk of mortality between infants with PTB-SGA, PTB without SGA, term birth (TB) with SGA (TB-SGA) and TB without SGA. The study defines PTB as occurring between 23-36 weeks’ gestational age (GA), and SGA as birth weight <5th percentile for sex/GA, rather than the more commonly used <10th percentile. Their data showed that when using TB neonates as a reference group, both the rate and relative risk of mortality were highest in the PTB-SGA group, followed by the PTB group and TB-SGA group. With few exceptions, this trend remained consistent when the authors adjusted for maternal factors including world region of origin, age, parity smoking status, income quintile and infant factors such as sex and chromosomal anomalies. The authors note limitations such as not adjusting for additional maternal factors that may cause intrauterine growth restriction (e.g. pre-pregnancy hypertension and maternal nutrition). For medical providers, this data supports further research and initiatives to reduce the incidence of both PTB and SGA.
In-Depth [retrospective cohort]: The final study cohort included 1 676 110 singleton births that occurred between April 1 2002 to March 3 2015 and were restricted to 23-42 weeks’ gestation. Data was obtained primarily from the Canadian Institute for Health Information’s Discharge Abstract Database. The primary outcome was neonatal death <28 days from birth. The TB group was used as a reference group and had a mortality rate of 0.6 per 1000 infants. The TB-SGA group had a mortality rate of 2.8 per 1000 (aRR=4.6; 95%CI=35.4-41.4). The PTB without SGA group had a mortality rate of 22.9 per 1000 (aRR=38.3; 95%CI=35.4-41.4). The PTB-SGA group had a mortality rate of 60 per 1000 (aRR=96.7; 95%CI=85.4-109.5). This pattern largely persisted when data was stratified for maternal demographic factors. Stratifying the main model by maternal world region of origin showed higher absolute rates for the Carribean and Sub-Saharan African group (approximately 83 per 1000), while South Asians (30 per 1000) and Canadians (52 per 1000) had the lowest absolute rates. A final analysis where births were restricted to 24-42 weeks’ GA and there were no chromosomal anomalies also showed persistence of the neonatal mortality pattern. In sub-analysis where SGA was categorized as 23-31 versus 32-36 weeks’ GA, mortality was the highest in the 23-31 week group (aRR=471.6;95%CI=416.1-534.4) compared to the 32-36 week group (aRR=34.9; 95%CI=28.3-43)
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