Having an Apgar score below 7 at five minutes was associated with greater prevalence of neurologic disability and of low cognitive function among Danish draftees. An Apgar score <7 may be a marker for severity of neurologic impairment, as suggested by its inverse association with proportion of disqualifying neurologic diagnoses. The absolute risk increases of 6.6% for neurologic disability and 9.9% for low cognitive function associated with five-minute Apgar score <7 imply a low sensitivity and thus a limited clinical utility in predicting long-term disability. At the same time, we found limited evidence of the absolute increase in risk of low cognitive function rising to 15%–35% if Apgar score <7 was accompanied by very young maternal age, growth restriction, or instrument delivery. Accounting for measured perinatal characteristics attenuated but did not fully account for the observed associations. The results indicate that low Apgar score is associated with impaired neurodevelopment through several mechanisms, only some of which involve clinical characteristics typically observed at birth. Five-minute Apgar scores in the 7–9 range were also associated with worse outcomes in our data, which is consistent with the notion of gradual increase in risk with worsening of condition at birth.
Our findings regarding Apgar score and neurologic disability are in agreement with recent reports of an inverse association between five-minute Apgar score and long-term risk of epilepsy [16, 17]. Unlike those studies, which ascertained epilepsy from computerized hospitalization records, we used diagnoses reported directly to physicians, which should be less likely to include false-positive diagnoses . Our findings regarding cognitive function corroborate and extend existing knowledge. Odd et al., in a population of >130,000 Swedish draftees born in early 1970s, found a small reduction in mean IQ scores associated with low Apgar scores, similar in magnitude to our findings. The estimate of risk ratio for poor cognitive function in our study, 1.33 (0.94–1.88), and that from the study of Odd et al., 1.35 (95% CI: 1.07–1.69)  were similar despite the latter study defining poor cognitive score as the bottom 9% of the distribution. In clinical practice, cutoffs of <1 and <2 standard deviations below the mean IQ are commonly used. In our data, prevalence ratios for those two definitions of low cognitive function associated with Apgar score <7 were, respectively, 1.44 (95% CI: 1.00–2.07) and 1.74 (95% CI: 0.80–3.81). These outcomes were not used in the main analyses because of small number of 'events' in the Apgar <7 group. Lawlor et al. found a 5-minute Apgar score <8 to be associated with a 1.6-point mean decrease in IQ among adolescents at age 14 years , a value similar to our estimate of a 1.8-point mean decrease. Seidman et al. found a nearly null association among Israeli conscripts  based on the examination of mean differences. We do not interpret a mean IQ decrease on the order of one-tenth of one standard deviation as being clinically significant. On the other hand, a small population shift could reflect a more important deficit for subgroups of a population. By analogy, small shifts in the mean blood pressure of a population may be important for specific subgroups . Thus, examining only mean difference may mask effects seen only in the fringes of the total distribution.
We estimated potential loss to follow-up from the underlying birth cohort in a sample of 14,288 boys born in the study area in 1980–1983. Before reaching conscription age, the emigration out of Denmark was <1%; and mortality was 1.2%. For boys with five-minute Apgar scores below 7, mortality before age 1 year was 28%, most of the deaths occurring among boys with scores below four. The overall mortality before age 1 year was 0.8%, which is consistent with the period nationwide estimates . We had no information on potentially eligible men who may have been institutionalized for legal or health reasons. Thus, at conscription age, men with a history of low Apgar score appearing before the draft board represent a comparatively healthy subset of all newborns with low Apgar scores. As noted by Odd et al., the differential survival underscores the fact that associations between neonatal condition and adult neurodevelopment seem to persist even in relatively healthy men .
Misclassification of the newborn condition by recorded Apgar score could result from random data entry errors and from the partially subjective nature of the Apgar score. These errors can be assumed to be independent of the outcomes we examined and therefore unlikely to cause upward bias in our measures of association. Neurologic diagnoses among conscripts may be under-ascertained by the draft-file information, if only the first-reported disqualifying diagnosis is recorded. Nevertheless, the prevalence of neurologic conditions in our sample (2.2%) is comparable with published population figures . Accounting for measured perinatal characteristics caused prevalence ratios to decrease, implying that using better-measured or additional factors theoretically could reduce the observed associations. For example, accounting for socioeconomic markers other than maternal characteristics could explain part of the associations with cognitive function, as it did the Swedish cohort .