Introduction: Late-preterm birth (34-36 weeks' gestation) has been associated with a risk for long-term cognitive and socioemotional problems. However, many studies have not incorporated measures of important contributors to these outcomes, and it is unclear whether effects attributed to gestational age are separate from fetal growth or its proxy, birth weight for gestational age.
Method: Data came from a study of low- and normal-weight births sampled from urban and suburban settings between 1983 and 1985 (low birth weight, n = 473; normal birth weight; n = 350). Random sampling was used to pair singletons born late-preterm with a term counterpart whose birth weight z score was within 0.1 SD of his or her match (n = 168 pairs). With random-effects models, we evaluated whether pairs differed in their IQ scores and teacher-reported behavioral problems at the age of 6 years.
Results: In adjusted models, late-preterm birth was associated with an increased risk of full-scale (adjusted odds ratio [aOR]: 2.35 [95% confidence interval (CI): 1.20-4.61]) and performance (aOR: 2.04 [95% CI: 1.09-3.82]) IQ scores below 85. Late-preterm birth was associated with higher levels of internalizing and attention problems, findings that were replicated in models that used thresholds marking borderline or clinically significant problems (aOR: 2.35 [95% CI: 1.28-4.32] and 1.76 [95% CI: 1.04-3.0], respectively).
Conclusions: Late-preterm birth is associated with behavioral problems and lower IQ at the age of 6, independent of maternal IQ, residential setting, and sociodemographics. Future research is needed to investigate whether these findings result from a reduction in gestational length, in utero (eg, obstetric complications) or ex-utero (eg, neonatal complications) factors marked by late-preterm birth, or some combination of these factors.