Objective
Late-preterm infants (34 weeks 0/7 days-36 weeks 6/7 days’ gestation) represent the largest proportion of singleton preterm births. A systematic review was performed to access the short- and/or long-term morbidity of late-preterm infants.
Study Design
An electronic search was conducted for cohort studies published from January 2000 through July 2010.
Results
We identified 22 studies studying 29,375,675 infants. Compared with infants born at term, infants born late preterm were more likely to suffer poorer short-term outcomes such as respiratory distress syndrome (relative risk [RR], 17.3), intraventricular hemorrhage (RR, 4.9), and death <28 days (RR, 5.9). Beyond the neonatal period, late-preterm infants were more likely to die in the first year (RR, 3.7) and to suffer from cerebral palsy (RR, 3.1).
Conclusion
Although the absolute incidence of neonatal mortality and morbidity in infants born late preterm is low, its incidence is significantly increased as compared with infants born at term.
The incidence of preterm birth, defined as delivery before the end of the 37th week (259th day) of pregnancy from the first day of the last menstrual period, is increasing. In the United States, the preterm rate rose from 9.1% in 1981 to 12.3% in 2003. In certain regions in Brazil the prevalence of preterm birth was 15% according to the 2004 Pelotas birth cohort, roughly 3 times the prevalence found in the 1982 birth cohort in the same city.
For Editors’ Commentary, see Table of Contents
Infants born between the gestational ages of 34 weeks and 0/7 days through 36 weeks and 6/7 days (239th-259th day) are called near term or late preterm. Late-preterm infants account for about 74% of all preterm births and about 8% of all births. They are recognized as the fastest-increasing and largest proportion of singleton preterm births. This increase might be due to a perception that electively delivered late-preterm babies face few risks.
Several recent studies of late-preterm infants have documented increased short-term medical risks during their birth hospitalizations and increased adverse long-term outcomes (medical, social, behavior, school performance) compared to full-term infants. Nevertheless, short- and long-term outcomes of late-preterm infants are not as frequently described as the outcomes of extremely preterm newborns and infants born late preterm are usually not entered in long-term developmental follow-up programs.
The aim of the current study was to perform a systematic review of the literature for medical and developmental short- and long-term outcomes of late-preterm infants to describe morbidity associated with late-preterm birth.
Materials and Methods
Search strategy
We performed an electronic search in PubMed, MEDLINE, Embase, and Cochrane trials databases (inception from January 2000 through July 2010) for original (cohort) studies that reported on short-term and/or long-term outcomes of infants born late preterm. The search parameters we used were “34 weeks” or “35 weeks” or “36 weeks” and “late preterm” or “near term” and “complications” or “morbidity” or “outcome.” To be included, a study had to report on the short- and/or long-term outcomes of late-preterm infants (34-36 weeks 6/7 days) compared to full-term infants (≥37 weeks). Reference lists of known articles were checked to identify cited articles not captured by electronic searches. Review articles were also excluded but their reference lists were screened for relevant studies. Cohort studies reporting on <50 infants were excluded. There were no language restrictions.
Data extraction
The following data were extracted from included articles: author, year of publication, methodological characteristics of each study, sample size, and short- and/or long-term outcomes. Short-term outcomes included neonatal outcomes such as Apgar score, need for mechanical ventilation or intubation, nasal continuous positive airway pressure, use of nasal oxygen, use of surfactant, presence of transient tachypnea, respiratory distress syndrome, persistent pulmonary hypertension, apnea, pneumothorax, pneumonia, meningitis, sepsis, hypoglycemia, feeding problems, hypothermia, hyperbilirubinemia, jaundice requiring phototherapy, and neonatal death. Long-term outcomes incorporated complications such as neurological morbidity, school performance, growth, and social outcomes. All articles were scored independently by 2 reviewers; disagreement was resolved by consensus or by a third reviewer.
Statistical analysis
All extracted information was systematically recorded in a database, in which we classified methodological characteristics of each study and their outcomes. For each outcome, we calculated the absolute risk (AR) of neonatal outcome and relative risk (RR) with corresponding 95% confidence intervals (CIs). Heterogeneity was explored by Cochrane Q 2 test and I 2 . I 2 can be interpreted as the proportion of total variation observed between the trials attributable to differences between trials rather than sampling error (chance). I 2 >75% is considered as a heterogeneous metaanalysis. We used a random effects model for pooling RR. Review Manager 5.0 (The Cochrane Collaboration, 2008; www.cochrane.org ) was used to calculate these pooled effect estimates (RR and 95% CI).
Results
The results of the search strategy are shown in Figure 1 . The electronic search detected 314 articles of which 48 were selected for full reading after studying the abstract. From these 48 articles, 8 studies were excluded because there was no full-term comparison group included. Six studies were excluded because they included infants born <34 weeks of gestation in their analysis. Twenty studies were excluded for other reasons (eg, studies reporting on <50 infants, review articles, not possible to make 2×2 tables). From references in selected articles and identified reviews, another 8 articles were included. Thus, 22 articles were available for the final review. These 22 studies included in total 2,368,471 late-preterm infants and 27,007,204 term infants. The results of these 22 studies were used to calculate pooled RR and 95% CI if possible. For some outcomes high values of I 2 (>75%) were found when calculating pooled RR. Nevertheless, almost all studies reporting on a specific outcome showed the same direction of effect (RR, >1). Characteristics of the included studies are presented in Table 1 .
| Study ID | Author | Year | No. of LPI | No. of FTI | Outcomes | GA LPI (wk d/d) | GA FTI (wk d/d) | Study design | Data collection | Exclusion of infants with congenital anomalies |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Wang et al | 2004 | 90 | 95 | Short term | 35 0/7-36 6/7 | 37 0/7-40 6/7 | Cohort | Retrospective | Yes |
| 2 | Bastek et al | 2008 | 69 | 134 | Short term | 34 0/7-36 6/7 | ≥37 | Cohort | Retrospective | Yes |
| 3 | McIntire et al | 2008 | 21,771 | 80,014 | Short term | 34 0/7-36 6/7 | 37 0/7-39 6/7 | Cohort | Retrospective | Yes |
| 4 | Yoder et al | 2008 | 895 | 12,905 | Short term | 34 0/7-36 6/7 | 37 0/7-40 6/7 | Cohort | Retrospective | No |
| 5 | Guasch et al | 2009 | 2003 | 32,015 | Short term | 34 0/7-36 6/7 | 37 0/7-42 6/7 | Cohort | Retrospective | No |
| 6 | Kalyoncu et al | 2010 | 252 | 252 | Short term | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | Yes |
| 7 | Kitsommart et al | 2009 | 1193 | 8666 | Short term | 34 0/7-36 6/7 | ≥37 | Cohort | Retrospective | No |
| 8 | Lubow et al | 2009 | 149 | 150 | Short term | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | No |
| 9 | Ma et al | 2009 | 2032 | 6867 | Short term | 34 0/7-36 6/7 | ≥37 | Cohort | Prospective | No |
| 10 | Melamed et al | 2009 | 2478 | 7434 | Short term | 34 0/7-36 6/7 | 37 0/7-40 0/7 | Cohort | Retrospective | Yes |
| 11 | Hibbard et al | 2010 | 19,334 | 165,993 | Short term | 34 0/7-36 6/7 | 37 0/7-40 6/7 | Cohort | Retrospective | No |
| 12 | Tomashek et al | 2007 | 2,221,545 | 24,973,117 | Short term + 12 mo | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | No |
| 13 | Young et al | 2007 | 21,106 | 247,433 | 12 mo | 34 0/7-36 6/7 | 37 0/7-40 6/7 | Cohort | Retrospective | No |
| 14 | Santos et al | 2008 | 447 | 3262 | Short term until 3 mo | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Prospective | No |
| 15 | Chyi et al | 2008 | 970 | 13,671 | Fifth grade | 34 0/7-36 6/7 | ≥37 | Cohort | Retrospective | No |
| 16 | Moster et al | 2008 | 32,187 | 853,309 | 20-36 y | 34 0/7-36 6/7 | ≥37 | Cohort | Prospective | Yes |
| 17 | Pulver et al | 2009 | 25,973 | 316,077 | Short term + 12 mo | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | No |
| 18 | Santos et al | 2009 | 371 | 2914 | 12 and 24 mo | 34 0/7-36 6/7 | 37 0/7-42 6/7 | Cohort | Prospective | No |
| 19 | Baron et al | 2009 | 60 | 35 | Short term + 3 y | 34 0/7-36 6/7 | ≥37 | Cohort | Retrospective | Yes |
| 20 | Morse et al | 2009 | 7152 | 152,661 | 3-5 y | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | Yes |
| 21 | Gurka et al | 2010 | 53 | 1245 | 15 y | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Prospective | Yes |
| 22 | Petrini et al | 2009 | 8341 | 128,955 | Short term + 5.5 y | 34 0/7-36 6/7 | 37 0/7-41 6/7 | Cohort | Retrospective | No |
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