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Previous Volume 339:1434-1439 November 12, 1998 Number 20 Next

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ABSTRACT

Background The rates of preterm birth have increased in many countries, including Canada, over the past 20 years. However, the factors underlying the increase are poorly understood.

Methods We used data from the Statistics Canada live-birth and stillbirth data bases to determine the effects of changes in the frequency of multiple births, registration of births occurring very early in gestation, patterns of obstetrical intervention, and use of ultrasonographic dating of gestational age on the rates of preterm birth in Canada from 1981 through 1983 and from 1992 through 1994. All births in 9 of the 12 provinces and territories of Canada were included. Logistic-regression analysis and Poisson regression analysis were used to estimate changes between the two three-year periods, after adjustment for the above-mentioned determinants of the likelihood of preterm births.

Results Preterm births increased from 6.3 percent of live births in 1981 through 1983 to 6.8 percent in 1992 through 1994, a relative increase of 9 percent (95 percent confidence interval, 7 to 10 percent). Among singleton births, preterm births increased by 5 percent (95 percent confidence interval, 3 to 6 percent). Multiple births increased from 1.9 percent to 2.1 percent of all live births; the rates of preterm birth among live births resulting from multiple gestation increased by 25 percent (95 percent confidence interval, 21 to 28 percent). Adjustment for the determinants of the likelihood of preterm birth reduced the increase in the rate of preterm birth to 3 percent among all live births and 1 percent among singleton births.

Conclusions The recent increase in preterm births in Canada is largely attributable to changes in the frequency of multiple births, obstetrical intervention, and the use of ultrasound-based estimates of gestational age.

Source Information

From the Bureau of Reproductive and Child Health, Laboratory Centre for Disease Control, Ottawa, Ont. (K.S.J., S.W.W.); the Department of Pediatrics and Department of Epidemiology and Biostatistics, McGill University Faculty of Medicine, Montreal (M.S.K, R.P.); the Laval University Epidemiology Research Group, Quebec, Que. (S.M.); the Department of Newborn and Developmental Paediatrics, Women's College Hospital, Toronto (A.O.); and the Division of Neonatal–Perinatal Medicine, Dalhousie University, Halifax, N.S. (A.A.) — all in Canada.

Address reprint requests to Dr. Joseph at the Bureau of Reproductive and Child Health, Laboratory Centre for Disease Control, Tunney's Pasture AL 060 1E2, Ottawa, ON K1A OL2, Canada.

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