This study evaluated the consequences of late preterm birth on child growth in the first two years of life as compared to term deliveries in a population-based cohort from a middle-income country. Among the strengths of the study, besides sample size, are the low rates of refusal and loss to follow-up, and the prospective cohort design that allows assessment of temporal relationships.
A key limitation of the study is that most of the confounders studied were self-reported by mothers. Furthermore, in the analyses of late preterm birth and wasting at 12 and 24 months of age, the low number of children born late preterm that were wasted resulted in reduced precision.
Some methodological issues of this study are worthy of being discussed. First, some of the increase in morbidity among late preterm children may be attributable to observation and detection bias, because mothers and medical doctors may be more attentive to monitor symptoms and signs of medical complications and diseases in preterm than in term infants. However, higher hospitalization rates indicate that late preterm children are at increased risk of developing more severe illnesses than term children [12
]. A small number of studies have investigated short-term rehospitalization rates (2-4 weeks of discharge from the birth hospitalization) among late preterm children [12
]. These studies all reported higher rehospitalization rates among late preterm children than in term infants. We were able to locate only two published studies about rehospitalization among late preterm children after the neonatal period [15
]. Escobar et al found that gestational age of 36 weeks was a predictor of elevated risk for hospitalization within 15 to 182 days after discharge [15
]. McLaurin et al showed that late-preterm infants, whether discharged early (<4 days from the date of birth) or late after birth, were almost twice as likely as term infants to have been rehospitalized at some time during their first year of life [16
]. Severity of illness leading to hospitalization is also reflected in the increased risk of mortality among preterm as compared to term infants. Previous analyses of our cohort showed that adjusted relative risks for neonatal and infant mortality, respectively, were 5.1 (1.7- 14.9) and 2.1 (1.0, 4.6) times higher for late-preterm children than for term children [17
Second, in the study of McLaurin et al [16
], bronchiolitis due to respiratory syncytial virus and pneumonia were the most common causes of rehospitalization during the first year among both late-preterm and term infants. Although immaturity of the respiratory system may put late preterm children at higher risk of more severe community-acquired pulmonary infections, part of the hospitalizations may be due to Berkson bias since late preterm births were more prevalent among low income families.
Third, it has already been shown that preterm infants and children born small for gestational age exhibit different rates of growth in early childhood [18
]. Prematurity and IUGR frequently present together in the same newborn. The exclusion of small-for-gestational age newborns from the current study aimed to prevent this potential confounding effect.
The importance of analyzing the effect of late preterm births on growth indicators is supported by the marked increase in preterm births observed across the three population-based perinatal studies carried out in Pelotas in 1982, 1993 and 2004. Preterm births increased from 6.3% in 1982 to 11.4% in 1993, and 14.7% in 2004 [19
]. Nearly two out of three preterm births in 2004 occurred at late-preterm gestations. The present study showed that the nutritional risks expressed in growth failure linked to late prematurity during infancy are extended to early childhood, at least until the age of two years.
Despite growing faster, late preterm children were at increased risk of underweight and stunting during the first two years of life. The strength of the association between late preterm birth and stunting was similar for the first and the second years with an almost two-fold increase in risk among late preterm children as compared to children born at term. Wasting was associated with late preterm birth only in the first year of life, although the small number of wasted children in both follow-ups may be responsible for the lack of association in the second year of life. In fact, only 4 (1.1%) late preterm and 7 (0.3%) term children showed signs of wasting at 12 months. At 24 months, there were 3 (0.8%) and 10 (0.4%) late preterm and term children, respectively, with signs of wasting. We found no studies that have evaluated the long-term nutritional status of late preterm children. Thus, we know neither the prevalence rates nor risk estimates for wasting and stunting in children from other populations. Previous studies suggest that the gastrointestinal tract is likely to be less developed in late preterm children [3
], which may lead to difficulties in sucking and swallowing, delay in successful breastfeeding, poor weight gain, and dehydration during early postnatal weeks [21
Late preterm children and term children gained on average 9 kg in weight from birth to 24 months of age, with a statistically non-significant advantage for late preterm children (who gained on average 13.23 g more than term children). Mean child length at 24 months was of 85.70 cm for late preterm children and 87.11 cm for term children, indicating that those born at term had on average a higher stature at the age of two years. However, linear growth from birth to 24 months of age was higher among late preterm children than among term children: although still shorter than term children at 24 months, late preterm children grew on average 1.26 cm more than term children.
Late preterm children gained as much as weight as their term counterparts and they grew more linearly than the term children in both follow-ups. Their growth, however, was not enough to prevent underweight and stunting in the first and second year of life. Analyses from the 1982 Pelotas Birth Cohort [18
] showed that despite their earlier disadvantage, preterm children gradually caught up with their appropriate birth weight, term counterparts. This catch-up occurred primarily between mean ages 23 and 47 months. As a result, it is possible that the growth failure observed in late preterm children from the 2004 cohort will be reversed in the coming two years.
There is considerable evidence that size early in life and growth pattern in fetal life and infancy are inversely associated with chronic disease in late childhood and adolescence. Components of growth during childhood, other than body size at birth or later on, is being explored as a mechanism in the development of obesity and other chronic diseases [24
]. Results from long-standing cohort studies in Brazil, Guatemala, India, the Philippines, and South Africa showed that lower birth weight and stunting in the first two years of life were risk factors for high glucose concentrations, blood pressure, and adverse lipid profiles once adult body-mass index and length were adjusted for [25