Birth weight and longitudinal growth in infants born below 32 weeks' gestation: a UK population study

Abstract

Objective: To describe birth weight and postnatal weight gain in a contemporaneous population of babies born <32 weeks' gestation, using routinely captured electronic clinical data.

Design: Anonymised longitudinal weight data from 2006 to 2011.

Setting: National Health Service neonatal units in England.

Methods: Birth weight centiles were constructed using the LMS method, and longitudinal weight gain was summarised as mean growth curves for each week of gestation until discharge, using SITAR (Superimposition by Translation and Rotation) growth curve analysis.

Results: Data on 103 194 weights of 5009 babies born from 22-31 weeks' gestation were received from 40 neonatal units. At birth, girls weighed 6.6% (SE 0.4%) less than boys (p<0.0001). For babies born at 31 weeks' gestation, weight fell after birth by an average of 258 g, with the nadir on the 8th postnatal day. The rate of weight gain then increased to a maximum of 28.4 g/d or 16.0 g/kg/d after 3 weeks. Conversely for babies of 22 to 28 weeks' gestation, there was on average no weight loss after birth. At all gestations, babies tended to cross weight centiles downwards for at least 2 weeks.

Conclusions: In very preterm infants, mean weight crosses centiles downwards by at least two centile channel widths. Postnatal weight loss is generally absent in those born before 29 weeks, but marked in those born later. Assigning an infant's target centile at birth is potentially harmful as it requires rapid weight gain and should only be done once weight gain has stabilised. The use of electronic data reflects contemporary medical management.

Keywords: Data Collection; Growth; Neonatology; Statistics.

Figure 1

Birth weight by gestation in 2679 boys and 2330 girls.

Figure 2

Centiles of birth weight by gestation and sex, with the British 1990 0.4th, 50th and 99.6th centiles superimposed (sexes averaged).

Figure 3

Growth curves of weight by postmenstrual age in 1012 infants born at 31 weeks’ gestation. The curves are shown (left) unadjusted and (right) adjusted using each infant's SITAR (Superimposition by Translation and Rotation) random effects. See text for details.

Figure 4

Growth curves of weight by postmenstrual age in 176 infants born at 22–23 weeks’ gestation. The curves are shown (left) unadjusted and (right) adjusted using each infant's SITAR (Superimposition by Translation and Rotation) random effects. See text for details.

Figure 5

Mean growth curves of weight by postmenstrual age and week of gestation, superimposed on the British 1990 birth weight reference.

Figure 6

Mean growth curves of weight by postmenstrual age and week of gestation, superimposed on the British 1990 birth weight reference. Weight is plotted on a log scale, so the slope of each curve indicates relative growth and the velocity fan gives the slopes in g/kg/d.