Alberta birth certificate

ABBREVIATION. CI, confidence interval.

Recent increases in maternal age and in the use of ovarian stimulation and in vitro fertilization have led to increases in the frequency of twins and triplet and higher-order multiple births. (1-4) In Canada, multiple births increased in frequency from 18.2 per 1000 total births in 1974 to 19.3 in 1980, 21.2 in 1990, and 25.3 in 1997, (5-8) while twin births increased from 17.9 per 1000 total births in 1974 to 18.9 in 1980, 20.5 in 1990, and 24.2 in 1997. (5-8) Because twin births constitute over 95% of multiple births, most of the absolute increase in multiple births can be attributed to increases in twins, although on a relative scale, triplet and higher-order multiples have increased to a far greater extent.

Over the same time period, there has been a substantial increase in the rate of preterm birth (<37 weeks' gestation) among multiple births. (9,10) In Canada, the rate of preterm birth among multiple births increased from 33% in 1974 to 40% in 1981 to 1983, 50% in 1992 to 1994, and 53% in 1997. (5-8) The mean gestational age of twin live births has decreased from 36.5 weeks in 1981 to 1983 to 35.8 weeks in 1992 to 1994.9 Similarly, in the United States preterm birth rates among twins have increased from 41% in 1981 to 55% in 1997. (10)

This large increase in preterm birth among multiple births contrasts with a modest increase in preterm birth among singletons (increases of 25% vs 5% among multiple births vs singletons, respectively, between 1981-1983 and 1992-1994). (9) Although the increase in preterm birth among both singletons (11) and multiple births (10,12) seems to be primarily attributable to increases in preterm induction/cesarean section, the increase in preterm birth was associated with a decline in fetal mortality among singletons but not among multiple births. (9) If increased obstetric intervention is saving compromised singleton fetuses through early delivery, it is unclear why this anticipated benefit is not evident among multiple births, given the rising rates of preterm birth. Furthermore, although infant mortality among both singletons and twins has decreased over the period when preterm birth rates have increased, (8,10) it is unclear whether the decline in infant mortality among twins has been related to obstetric intervention or to other factors, such as the temporal decline in sudden infant death syndrome. We therefore conducted a study to examine changes in gestational age-specific, birth weight-specific, and cause of death-specific fetal and infant mortality among twin births in Canada in relation to concurrent increases in preterm birth.

METHODS

We used data from the live-birth and stillbirth databases of Statistics Canada for the years 1985 to 1996 and data from the mortality database for the years 1985 to 1997. Information in these databases is obtained from live birth, stillbirth, and death registrations. Information on gestational age in the live birth and stillbirth registrations is obtained from the responsible physician or the mother. The live birth and mortality databases were linked using a previously validated probabilistic methodology (13,14) to obtain perinatal information on infant deaths. The linkage process involved a comparison of fields common to records within both databases, an assignment of weights based on the closeness of the ensuing matches, and a global assessment of the likelihood of a valid match. Relevant birth and death registration documents were examined to resolve tentative links. The completeness and validity of the linked data has been studied for deaths that occurred in 2 provinces (infant deaths in Nova Scotia and neonatal deaths in Alberta) using information from hospital sources; 99% of such deaths were located within the linked files. (15)

The linked file was subjected to data quality checks and procedures to eliminate duplicate records. Births to mothers residing in Ontario were excluded from the analysis because of previously documented problems with data quality (16) and significant numbers of unlinked deaths. Births from Newfoundland were also excluded because data from this province are not available before 1991. In common with other large databases, we observed that a few live births with a birth weight of <500 g and a gestational age of <22 weeks seemed to have survived infancy. Given the extremely low probability of survival at this birth weight and gestational age, (17) we assumed that survival was attributable to a missing death certificate. Correspondingly, we reclassified the survival status of such live births to death on the first day of life; such reclassification of survival status is identified in all analyses.

We examined stillbirth and infant mortality rates within specific categories of gestational age and birth weight; the categories were created with the intention of providing reasonable homogeneity with regard to prognosis, while increasing the statistical stability of the estimates. Causes of death, classified in the databases with International Classification of Diseases, Ninth Revision codes, were also examined to help elucidate those causes responsible for reductions in mortality. Given the focus on stillbirth and infant death rates, each twin birth was considered separately (rather than as a twin set). Thus, gestational age-specific stillbirth rates were estimated per 1000 fetuses at risk in any particular gestational age category (ie, all fetuses stillborn or live born at that or a later gestational age).

The time span of the study was divided into 4 equal periods and time trends were assessed first by contrasting event rates in the earliest versus most recent periods (1985-1987 vs 1994-1996) and also by estimating trends over the 4 time periods. Contrasts between periods were made by computing relative risks, relative risk reductions, and their 95% confidence intervals (CIs), while trends over time were assessed using [chi square] tests for trends in proportions. Logistic regression, with stillbirth or infant death as the outcome, was used to examine whether period changes in mortality could be attributed to simultaneous changes in the distribution or risk of determinants such as maternal age, parity, and birth order. In a supplementary analysis, we repeated the statistical assessment of the primary temporal contrasts (preterm birth, low birth weight, stillbirth, stillbirth [greater than or equal to] 34 weeks, and infant mortality in 1985-1987 vs 1994-1996) using the procedure of generalized estimating equations (18) to adjust the variance estimates for the correlation in the outcomes of a single twin pregnancy. Because our data source did not identify twin sets, we created sets using identifiers such as birth date, maternal age, and province of residence (approximately 13 800 sets for 28 400 twin births).

RESULTS

The rate of preterm birth among twin live births increased significantly by 17% (95% CI: 14%-20%, P < .001), from 42.5% in 1985 to 1987 to 49.6% in 1994 to 1996. Most of this increase was attributable to increases in twin live births between 34 and 36 weeks' gestation; such live births increased by 23% (95% CI: 19%-28%) from 27.1% in 1985 to 1987 to 33.3% in 1994 to 1996. There was a smaller 8% (95% CI: 0%-18%) increase in live births at 32 to 33 weeks from 6.7% of twin live births in 1985 to 1987 to 7.3% in 1994 to 1996 (P = .06). Figure 1 shows the overall "shift to the left" in the gestational age distribution of twin live births. In contrast, low birth weight rates were essentially stable, registering a modest increase of 2% (95% CI: 0%-4%, P = .07), from 48.5% in 1985 to 1987 to 49.6% in 1994 to 1996. However, live births <500 g increased in frequency from 5.4 per 1000 twin live births in 1985 to 1987 to 7.6 in 1994 to 1996 (42% increase, 95% CI: 5%-91%, P value for trend = .004).

[FIGURE 1 OMITTED]

Stillbirth rates among twins declined from 22.4 per 1000 total births in 1985 to 1987 to 18.7 per 1000 in 1988 to 1990, 20.3 per 1000 in 1991 to 1993 and 18.8 per 1000 total births in 1994 to 1996. The 16% (95% CI: 1%-28%) overall decline in stillbirth rates between 1985 and 1987 and 1994 and 1996 was significant (P = .03), although the pattern of the decrease was not consistent (P for linear trend = .09, Table 1). Gestational age-specific stillbirth rates increased significantly at 22 to 23 weeks' gestation and declined significantly at 34 to 36 and 37 to 41 weeks' gestational age (Fig 2). Among fetuses reaching 34 or more completed weeks' gestation, stillbirth rates decreased from 9.5 per 1000 in 1985 to 1987, to 7.7 per 1000 in 1988 to 1990, 6.8 per 1000 in 1991 to 1993, and 5.4 per 1000 fetuses at risk in 1994 to 1996 (P for trend <.001, Fig 2). This represents a 43% (95% CI: 23%-58%, P < .001) decrease in stillbirths between 1985 and 1987 and 1994 and 1996 among fetuses reaching [greater than or equal to] 34 weeks' gestation.