Description of the Studies
Among the 1774 studies identified in the database research, 32 matched the eligibility criteria (Figure 1). Of these studies, 27 had a cohort design, 2 had a cross-sectional design, 2 case-controls, and 1 was a randomized clinical trial.
18 studies had a full-term control group, 12 described the prevalence in the extremely preterm group, 17 described the prevalence in the very preterm group, and 8 described the prevalence in the moderate/late preterm group. Only 6 studies described more than one prematurity group according to gestational age, and 2 of them were classified by birth weight instead of gestational age, thus were excluded from the gestational age analysis.