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Breast milk attenuates poor postnatal growth in male preterm infants

Presented at the Neonatal Society 2004 Summer Meeting (programme).

Cockerill J1, de Vega I2, Doré C2, Uthaya S1, Modi N1

1 Imperial College London, London, UK
Hammersmith Hospitals Trust, London, UK

Background: Extremely preterm birth is an acknowledged risk factor for poor growth. Current clinical practice aims to promote postnatal weight gain and head growth. Male infants are known to achieve less well than females over a range of outcomes, but there has been little comparative evaluation of growth.

Aims: The aim of this study was to compare the pattern of weight gain and head growth to age term-equivalent in male and female infants born = 32 weeks gestational age (GA) and to relate these to breast milk received and illness severity.

Methods: This study was approved by the hospital research ethics committee. We extracted the following information from the Queen Charlotte's & Chelsea Hospital neonatal database on all inborn infants ≤ 32 weeks GA who remained in the neonatal unit up to ≥ 37 weeks postmenstrual age and who had been admitted between 01/01/2002 and 31/12/2003: sex, head circumference (HC) and weight (Wt) at birth (B) and discharge (D), date of birth and discharge, antenatal and postnatal steroid exposure, number of days any breast milk was received and number of days receiving level 1 and 2 intensive care. We expressed each anthropometric index as standard deviation score (SDS) and growth between birth and 6 weeks as SDS gain (SDSG) calculated using commercially available software based on the 1990 British growth reference. SDSG is the change in SDS adjusted for sex and reference correlations between measurements at two time points. We expressed days of level 1 and 2 intensive care (%L1&2IC) and the number of days breast milk was received (%BM) as a percentage of the number of days from birth to discharge. Data were analysed using SPSS version 11.5. Unless otherwise stated, data are presented as mean (sd). The independent and paired samples t-test was used for between and within group comparisons; equal variances were not assumed. Linear and multiple regression analyses were used to compare weight and HC SDSG in male and female infants and to investigate the interaction between %BM and sex.

Results: There were 29 boys and 32 girls. There was no significant difference between boys and girls in GA, birth weight and HC, discharge HC, %BM or %L1&2IC (table). No infant received postnatal steroids and all but 5 (4 boys, 1 girl) received antenatal steroids.

There was a highly significant fall in weight SDS between birth and discharge in both boys (mean diff 1.28, p<0.0001) and girls (mean diff 0.98, p<0.0001). In contrast, head growth was maintained. No impact of %L1&2IC on growth was identified. Though not significantly different between boys and girls at birth, by discharge, boys had a significantly lower weight SDS and weight SDSG was more negative (table). Multiple regression analysis, allowing for BWt SDS, showed a significant positive relationship between weight SDSG and %BM in the boys (B=0.031; p=0.01) but not the girls (B=-0.009; p=0.23). There was a highly significant interaction between sex and %BM (p=0.007). Based on the mean BWt SDS for the whole group, and using the regression coefficients obtained from the analysis of each sex separately, the mean difference between boys and girls was 1.77 SDSG at 50%BM (p<0.0001), reducing to -0.22 SDSG at 100%BM (p=0.35).

Conclusion: Clinical management appears to have sustained postnatal head growth, in contrast to weight gain, in this 2002/2003 cohort of extremely preterm infants. Postnatal weight gain in boys is significantly worse than in girls but this difference is substantially reduced by breast milk.

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