A Health Surveillance Update on Canadian Children and Youth

Preterm Birth

Preterm birth has been identified as one of the most important perinatal health problems in industrialized nations and accounts for 75-85% of all perinatal mortality in Canada.

Preterm birth, defined as birth before 37 weeks' gestational age, has been identified as one of the most important perinatal health problems in industrialized nations.⁽⁷⁾ Preterm birth accounts for 75-85% of all perinatal mortality (stillbirths plus deaths to infants less than 7 days old) in Canada.⁽⁸⁾ The preterm birth rate is the number of preterm births per 100 live births in any given year. A related term is very preterm birth, which is defined as a gestational age less than 32 weeks.

In Canada in 1996, the preterm birth rate was 7.1 per 100 live births. Ontario is excluded from the Canadian preterm birth rate calculations because of concerns about the quality of the data.⁽⁹⁾

As depicted in Figure 3, preterm birth in Canada has been increasing gradually, from 6.6% of live births in 1991 to 7.1% of live births in 1996. Potential explanations for this increase include changes in the frequency and character of multiple-gestation pregnancies, increases in obstetric intervention, greater registration of early-gestation births and increases in the use of ultrasound-based estimates of gestational age.⁽⁹⁾

The availability of consistent and timely international comparisons of preterm birth rates is limited. Figure 4 compares the Canadian preterm birth rate with the 1996 rate in the United States⁽¹⁰⁾ and the 1995 rate in Australia.⁽¹¹⁾ The Canadian rate, while higher than the Australian rate, is considerably lower than the rate in the United States. A closer examination of the higher rate of preterm births in the United States reveals important racial differences in the preterm birth rates; preterm births occurred in 17.4% of live births to black women compared with 9.8% of live births to white women.⁽¹⁰⁾

Source: Bureau of Reproductive and Child Health, LCDC, based on Statistics Canada data⁽¹⁾

* excluding Ontario
Source: Bureau of Reproductive and Child Health, LCDC^{(1, 10, 11)}

Data limitations
An important limitation in the enumeration of preterm births is the potential for error in determining gestational age. Gestational age based on menstrual dates can be subject to considerable error due to inaccurate maternal reporting, the interpretation of post-conception bleeding as normal menses, irregular menstrual cycles and intervening, unrecognized pregnancy losses.⁽¹²⁾ It has been demonstrated that nearly 25% of infants who would be classified as preterm birth on the basis of the last normal menstrual period are not preterm. The validity of international comparisons is also limited by differences among nations in the completeness of birth registration and inconsistent ascertainment of gestational age.⁽¹²⁾

Summary
Preterm birth remains one of the main causes of perinatal mortality and morbidity.⁽¹³⁾ Careful analysis of the trends of preterm birth will ensure accurate interpretation of the occurrence and etiology of changing preterm birth rates over time.

Unless referenced otherwise, preterm birth statistics are the product of the Bureau of Reproductive and Child Health, LCDC.⁽¹⁾

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