International journal of epidemiology
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A case-control study of fatal venous thromboembolism in young women is described. Sixty women aged between 16 and 39 who died from thromboembolism in England and Wales between 1986 and 1988 were included in the study. Two living controls matched for age and marital status were sought from the records of the general practitioner with whom each case was registered. ⋯ These risks are considerably smaller than those observed in previous studies. The observed risk may be low because the dosage of oestrogen in modern oral contraceptive preparations has been reduced, but it may also be because the cases of fatal venous thromboembolism included in this study represent only a small proportion of all cases of venous thrombeombolism; a disease which is rarely fatal in young women. These results cannot necessarily be extrapolated to nonfatal venous thromboembolism.
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Seasonal variations in the proportion of preterm births in Japan from January 1979 to December 1983 are analysed using a traditional method of time-series analysis, which divides the variation in a series into trend, seasonal variation, other cyclic change, and remaining irregular fluctuations. It is shown that the proportion of preterm births in Japan have a clear seasonal periodicity with two peaks in summer and winter. Analysis of seasonality by period of gestation shows that interesting differences in kurtosis and skewness exist between summer and winter, i.e. the summer increase in preterm births was characterized by an increase of skewness which means an extension of the lower part of the distribution. ⋯ Theoretical simulations based on actual birth data in Japan over the period, are carried out to examine how season of conception could influence seasonal variations in the proportion of preterm births. Results show that, at least for first births, seasonality in conception rates could be one explanatory factor for the observed seasonal variation in proportions of preterm births. Another analysis reveals that conception in May and June are more likely to result in preterm births in Japan.