Editor—The Office for National Statistics uses an algorithm to classify neonatal deaths.1It is based on an hierarchical classification of causes to derive a single cause group for each death.2 Cause groups include, in descending order: congenital malformations, antepartum infection, immaturity related conditions, asphyxia/anoxia/trauma, external conditions, infections, other specific conditions, sudden infant deaths, and other unclassified conditions.

In applying the algorithm, we noted a surprisingly high frequency of deaths from malformations of the respiratory system, especially lung hypoplasia. This is one of the most common findings in neonates.3 Most cases are secondary to congenital malformations or pregnancy complications that inhibit lung development.4 As lung hypoplasia is secondary to preterm birth or premature rupture of membranes, it may be preferable to classify an infant death due to prematurity related lung hypoplasia under immaturity related conditions, rather than congenital malformations. We looked at how the reclassification of prematurity related lung hypoplasia deaths under immaturity related conditions, instead of congenital malformations, would affect the current hierarchical classification.

A detailed investigation of all 168 neonatal death records in 1993 with a code for lung hypoplasia (ICD 9: 7845) showed that 55 of these cases (32.7%) are secondary to immaturity related conditions (having one of the “immaturity” codes in the hierarchical classification) and have no other congenital malformations; 96 cases appear as secondary to other congenital malformations; and 17 appear as isolated lung hypoplasia or associated with other conditions such as hydrops fetalis. In the Office for National Statistics algorithm the 55 immaturity related cases would be classified as congenital malformation deaths because of the lung hypoplasia 7485 code. The cause groups for 1993 show that the total number of neonatal deaths classified under congenital malformations as the single cause is 1314.1 We conclude that a small percentage—that is, 4.2% or 55 of 1314—should preferably be classified as immaturity related conditions.