We identified all twins born in Scotland during 1981-4 from computerised maternity records. Subsequent admissions of twins to Scottish hospitals during 1981-94 were ascertained by probability matching on the basis of date of birth, sex, and surname. This matching is considered 99% accurate for singletons, but for twins it is reliable only at the level of the pair: which twin is admitted cannot be identified with certainty.
We identified hospital admissions for respiratory disease (ICD-9 (international classification of diseases, 9th revision) codes 464, 466, 480-486, and 490-496) for all Scottish children born during 1981-4. Rates of hospital admission among singletons and twins up to 10 years of age were compared by cause and sex, assuming Poisson errors in the numerators.
Twins were significantly less likely than singletons to be admitted for respiratory diseases (table). This was attributable to a reduced risk of admission for asthma among twins (code 493) by more than half throughout the age range 0-10 years. In contrast, twins were at significantly increased risk of admission for acute bronchitis and bronchiolitis (code 466). Admissions for other respiratory diseases were divided more equally between twins and singletons (table).
No significant differences were found between twins of the same or different sex in admission rates for any cause or all respiratory diseases combined. The relative difference in rates of admission with asthma for twins of the same sex compared with singletons was greater for males than for females, although this sex interaction was not significant.