Dear Editor,

In their study Drs Wright and Parkinson report that breast fed infants showed less postnatal weight gain compared to formula-fed infants and were significantly more likely to lose more than 10% of their birth weight.[1] They comment that this trend was no longer significant after adjustment for birth weight, which was significantly higher in breast fed infants. I wonder if adjusting for birthweight is actually obscuring a real difference.

Social and cultural factors including deprivation strongly influence the rates of maternal smoking, fetal growth retardation and breast-feeding. We have observed clear differences between the postnatal weight loss of breast and formula-fed infants.[2] It is possible that the reason for relatively smaller infants losing less weight may be that more are formula-fed. It might be more appropriate to test whether the relatively greater weight loss of larger infants persists after correcting for breast-feeding.

References

1. Wright CM and Parkinson KN. Postnatal weight loss in term infants: what is "normal" and do growth charts allow for it? Arch Dis Child Fetal Neonatal Ed 2004;89:F254.

2. Macdonald PD, Ross SRM, Grant L, Young D. Neonatal weight loss in breast and formula fed infants. Arch Dis Child Fetal Neonatal Ed 2003;88:F472.

Dear Editor,

In their study Drs Wright and Parkinson report that breast fed infants showed less postnatal weight gain compared to formula-fed infants and were significantly more likely to lose more than 10% of their birth weight. [1] They comment that this trend was no longer significant after adjustment for birth weight, which was significantly higher in breast fed infants. I wonder if adjusting for birthweight is actually obscuring a real difference. Social and cultural factors including deprivation strongly influence the rates of maternal smoking, fetal growth retardation and breast-feeding. We have observed clear differences between the postnatal weight loss of breast and formula-fed infants. [2] It is possible that the reason for relatively smaller infants losing less weight may be that more are formula-fed. It might be more appropriate to test whether the relatively greater weight loss of larger infants persists after correcting for breast-feeding.

References:

1. Wright CM and Parkinson KN. Postnatal weight loss in term infants: what is "normal" and do growth charts allow for it? Arch Dis Child Fetal Neonatal Ed 2004;89:F254.

2. Macdonald PD, Ross SRM, Grant L, Young D. Neonatal weight loss in breast and formula fed infants. Arch Dis Child Fetal Neonatal Ed 2003;88:F472.

Dear Editor

Wright and Parkinson show that by 5 days post-partum, weight is on average 50g less than at birth. This highlights a deficiency of current growth charts, which display weight increasing steadily with age. On the British 1990 chart for example, weight at 5 days is about 150g greater than at birth, a net discrepancy of 200g or two-thirds of a centile channel.

The obvious solution, to redraw the chart centiles just after term to reflect this dip in weight, does not work in practice. Infants of all gestations dip in weight after birth, not just those born at 40 weeks, so an infant born e.g. 5 days before term dips at the age that term infants are born. So a curve representing median weight by gestation at birth and at 5 days would need to consist of two separate curves, one about 50g above the other.

The ideal solution is to produce gestation-conditional centiles, i.e. separate charts for each week of gestation, each including the appropriate post-natal dip. But this is unrealistic for two reasons: not enough is known about the dip at each gestation, and too many separate charts would be needed (though a computer version would be feasible).

The simplest workable solution is to separate the centile curves for birthweight, i.e. up to 42 weeks gestation, from centiles for later weight, i.e. from 4 weeks post-term. The break in the curves should act as a reminder that before the break, weight in individual infants does not follow the centiles, whereas after the break it broadly does.