Growth of preterm babies after birth
Authors: Lex W Doyle
Journal: The Lancet Global Health (2015)

Assessment of the growth of children is fundamental to paediatrics. Aberrations of growth, whether too little or too much, can be signs of current or past ill health, particularly in the case of poor growth, or portend later ill health, particularly with excessive weight gain. To know whether growth is aberrant, accurate growth reference standards are required. Many standards exist for children born at term. Fewer are available for children born preterm and most have limitations, for example deficiencies in the participant selection or measurement techniques on which they were based. In The Lancet Global Health, José Villar and colleagues1Villar J Giuliani F Bhutta ZA et al.Postnatal growth standards for preterm babies: the Preterm Postnatal Follow-up Study of the INTERGROWTH-21stst Project: a multicentre population study.Lancet Glob Health. 2015; 3: e681-e691Summary Full Text Full Text PDF Scopus (194) Google Scholar describe the creation of growth curves for weight, length, and head circumference that are applicable to healthy singleton preterm babies. The study population was derived from a larger international study cohort of 4607 births for which the expected date of delivery was confirmed and fetal growth was measured extensively. Of these births, 224 (5%) were singleton preterm births. After exclusions, including for fetal growth restriction, 201 singleton neonates born at 26–36 completed weeks of gestation were enrolled into the INTERGROWTH-21st Project Preterm Postnatal Follow-up Study. Weight, length, and head circumference were measured by trained observers within 12 h of birth, every 2 weeks for 2 months, and then every 4 weeks until postnatal age 8 months. The major finding was that all measurements were lower in babies born between 33 and 36 weeks' gestation than those given in commonly used charts of size at birth for the same postmenstrual age in babies born at those gestational ages. By around 64 weeks' postmenstrual age (24 weeks' corrected age), however, values for all measures overlapped with those in the WHO Child Growth Standards for children born at term.2WHO Multicentre Growth Reference Study GroupWHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: methods and development. World Health Organization, Geneva2006Google Scholar The major contribution of the study is that it improves the estimates for postnatal growth for preterm infants, particularly for those born at 33–36 weeks' gestation. An important limitation of the study is that few babies born before 33 weeks' gestation could be included, largely because it becomes increasingly difficult to identify healthy babies with decreasing gestational age at birth. Infants born before 33 weeks' gestation comprise roughly 2% of all births, and 25% of all preterm births. Rather than trying to identify a cohort restricted to healthy babies, the methods could be replicated for all those born earlier than 33 weeks' gestation and charts constructed for those with and without major morbidities. Ehrenkranz and colleagues3Ehrenkranz RA Younes N Lemons JA et al.Longitudinal growth of hospitalized very low birth weight infants.Pediatrics. 1999; 104: 280-289Crossref PubMed Scopus (682) Google Scholar produced growth curves for weight for preterm babies in which those with major morbidity grew less well than those without. The main selection criterion for their study, however, was birthweight rather than gestational age. Another limitation of Villar and colleagues' study is the lack of growth data between birth and 2 weeks' postnatal age, when babies typically have substantial weight losses that increase as a percentage of bodyweight with diminishing gestational age at birth. They found no differential effect of lower gestational age on weight after birth, partly because the study was underpowered to detect such an effect, to which the lack of data in the first 2 weeks of life and the small number of very preterm babies contributed. Additionally, many preterm births involve multiple pregnancies—up to a third of all births before 28 weeks' gestation. Separate growth charts for babies born from multiple pregnancies could, therefore, be considered. Allowing for the limitations above, the study fills the gap between the accurate fetal growth curves reported from the same study group4Papageorghiou AT Ohuma EO Altman DG et al.International standards for fetal growth based on serial ultrasound measurements: the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project.Lancet. 2014; 384: 869-879Summary Full Text Full Text PDF PubMed Scopus (525) Google Scholar and the internationally adopted WHO Child Growth Standards that are appropriate from birth to age 5 years for singleton children born at term.2WHO Multicentre Growth Reference Study GroupWHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: methods and development. World Health Organization, Geneva2006Google Scholar Those two growth standards were developed with similar selection criteria to identify healthy singleton fetuses and children, respectively, under optimum environmental and nutritional conditions, and with similar rigorous methods to construct the growth curves, methods that also apply to the study of Villar and colleagues. With the postnatal growth curves of Villar and colleagues, fewer babies born preterm are likely to be classified as underweight than if charts of size at birth have been used previously. The proposed standards should not be extrapolated beyond the limits of Villar and colleagues' study, that is to babies born before 33 weeks, from multiple births, or who are unwell, or to data obtained in the first 2 weeks after birth. When applying growth curves to individuals, even those within the limits of the study, collection of data at multiple times will be most helpful, rather than just one or a few. Individual variation must always be considered as an explanation for deviations from the normal curves, rather than necessarily being due to a pathological basis. For researchers, the methods of the study, including the selection criteria for the pregnancies and babies, the standardised techniques for obtaining the measurements, and the data analysis, are an exemplar for those interested in investigating growth. How useful the charts might prove to be in clinical research will depend on the questions being addressed. For local practices, with respect to infant feeding or participant mixture, reality might vary substantially from the strict criteria used by Villar and colleagues and, therefore, researchers might not be able to avoid contemporaneous control groups in observational studies of preterm infants. The need for control groups in randomised trials with growth as a main endpoint will definitely not be avoidable. I declare no competing interests. Postnatal growth standards for preterm infants: the Preterm Postnatal Follow-up Study of the INTERGROWTH-21st ProjectOur data have yielded standards for postnatal growth in preterm infants. These standards should be used for the assessment of preterm infants until 64 weeks' postmenstrual age, after which the WHO Child Growth Standards are appropriate. Size-at-birth charts should not be used to measure postnatal growth of preterm infants. Full-Text PDF Open Access