Associations between birth interval <18 months (reference: 36-<60 months) and adverse neonatal and infant outcomes. SGA = Small-for-gestational-age, below the 10th percentile of a gender-specific reference distribution at each completed gestational week, using births in the US in 1991 . AGA = Appropriate-for-gestational-age. Preterm = below 37 completed weeks of gestation. *Reference: term-AGA.