DOI: 10.1111/j.1471-0528.2006.00923.x www.blackwellpublishing.com/bjog
General obstetrics
Preterm birth and multiple pregnancy in European countries participating in the PERISTAT project B Blondel,a A Macfarlane,b M Gissler,c G Breart,a J Zeitlin,a and the PERISTAT Study Group a INSERM,
U149, Epidemiological Research Unit on Perinatal and Women’s Health, Paris and Villejuif, France of Midwifery, City University, London, UK c National Research and Development Centre for Welfare and Health, STAKES, Helsinki, Finland Correspondence: B Blondel, INSERM, U149, 16 Avenue Paul Vaillant-Couturier, 94807 Villejuif Cedex, France. Email
[email protected] b Department
Accepted 9 February 2006.
Objective To compare rates of preterm birth among multiple
births in European countries, to estimate their contribution to overall preterm birth rates and to explore factors which could explain differences between preterm birth rates. Design Analyses of data from vital statistics, birth registers or
national samples of births. Setting Eleven member states of the European Union. Population All live births or representative samples of births at
national or regional level for the year 2000 or most recent year. Methods Description of rates of preterm birth before 37 and 32
weeks, estimation of population attributable risks (PAR), study of associations between preterm birth rates in multiples and singletons and nonspontaneous labour using Spearman’s rank correlation coefficient.
Results The proportion of multiple births before 37 weeks varied from 68.4% in Austria to 42.2% in the Republic of Ireland. In half of the countries, over 20% of all preterm births were attributable to multiple births. A strong association was found between the proportions of births before 37 weeks among multiple and singleton births (r = 0.81; P < 0.001). An association was observed between the rates of preterm birth and the proportions of deliveries with nonspontaneous onset among twins. Conclusions Wide variations in rates of preterm births and
deliveries with nonspontaneous onset were found between countries, suggesting marked differences in clinical practice which could have long-term implications for the health of children from multiple births. Keywords Caesarean, multiple, onset of labour, preterm birth.
Main outcome measures Preterm birth rates, PAR, proportions of
deliveries with nonspontaneous onset (caesarean sections before labour or induction of labour). Please cite this paper as: Blondel B, Macfarlane A, Gissler M, Breart G, Zeitlin J. Preterm birth and multiple pregnancy in European countries participating in the PERISTAT project. BJOG 2006; 113:528–535.
Patterns of multiple births and the context in which they occur have changed considerably since the 1970s. In most developed countries, multiple birth rates have risen mainly because of the increasing maternal age and the availability of fertility treatments.1 However, across Europe the timing and extent of the increase has varied. For example, between 1975 and 2002 the twinning rate increased by 50% in England and Wales and by 90% in the Netherlands.2 In addition to rising multiple birth rates, increase in preterm birth rates has been observed in countries such as Canada, the USA and France.3 As with multiple birth rates,
the size of this increase has varied between countries. It would appear that a substantial proportion of the increase may be attributed to a greater tendency for clinicians to intervene actively by inducing labour or undertaking a caesarean before labour, with the intention of preventing maternal or perinatal morbidity and mortality.4,5 To investigate this, we compared preterm birth rates among multiple births in the member states of the European Union, using aggregated data compiled for the PERISTAT project.6 Further, we wished to assess the risk of preterm birth attributable to multiple births in each country, and within the constraints of the available data, to attempt to explain differences in rates of preterm birth. For the last of these objectives,
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ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
Introduction
Preterm birth and multiple pregnancy in European countries
comparisons were also made between multiple and singleton births to test whether differences between member states were specifically related to multiple births.
Method The PERISTAT project was set up to develop a set of indicators for monitoring and describing perinatal health in Europe, forming part of the European Commission’s Health Monitoring Programme.6 After using a Delphi method to compile a list of indicators, a feasibility study was undertaken to assess the extent to which the participating countries were able to provide data to construct these indicators. Standard tables of aggregated data were derived from routine population-based data available in each country for the year 2000, if available, or the data for the most recent year were used.7 All states which were members of the European Union at the time of the project participated and provided data to a varying extent. In most countries, data were derived from civil registration or medical birth registers (Appendix). National data were available for seven countries (Austria, Denmark, Finland, Ireland, Italy, the Netherlands and Sweden). In France, rates of multiple births were available from birth registration but the other data were derived from the national perinatal survey, which included all births in 1 week in 1998. In Germany, data were available for all births in 9 of its 16 states (Bundesla¨nder), accounting for about 70% of all births in Germany. In the UK, data on gestational age were not available for England and Wales but data were provided separately for Scotland and Northern Ireland. For Belgium, only data from Flanders were used. Data for the Frenchspeaking area of Belgium and Greece were omitted because there were inconsistencies in the multiple births data. Portugal and Spain could not provide the relevant data for multiple births. In Luxembourg, the number of multiple births was too small to provide precise estimates of preterm birth rates. For the PERISTAT project, we obtained specific data on multiple births which were included in three of the tables requested from each country: 1. The number of women delivering live or stillborn babies by number of fetuses. 2. The number of live births subdivided into singleton births and multiples and gestational age at birth, in weeks. 3. The number of live births by method of onset of labour (spontaneous, induced and caesarean before labour) and whether they were singleton or twin preterm or term deliveries. Multiple maternities in each country were described in terms of maternities which were defined as pregnancies leading to one, two or more registrable births (live births or stillbirths). Rates of births before 32 weeks and between 33 and 36 weeks and the overall preterm birth rates before 37 weeks were described in terms of proportions of multiple live births. Rela-
ª RCOG 2006 BJOG An International Journal of Obstetrics and Gynaecology
tive risks (RR) and population attributable risks (PAR) for births before 37 weeks, with their confidence intervals,8 were calculated for multiples, with singletons as the reference group. The exact numbers of multiple live births were unknown for some countries. We therefore derived percentages of multiple live births from data about numbers of singleton and multiple maternities in the relevant country. Finally, we examined the associations between preterm births in multiples and two other indicators, preterm births in singleton births and deliveries with nonspontaneous onset. The latter were defined as caesarean sections before labour or inductions of labour. For the countries of the UK, the data related to elective caesareans, were defined as caesareans where the decision to intervene was taken before the onset of labour. These include a small but unknown number of caesareans, where a decision was taken before labour but the woman was already in labour when the caesarean section was performed. These deliveries with nonspontaneous onset were used as an indicator of the attitude to intervention at the end of pregnancy in each country, despite the fact that we had no information about whether the intervention was undertaken for clinical reasons, such as suspected fetal distress, or for nonmedical reasons. The ecological associations were tested using Spearman’s rank correlation coefficient.
Results Multiple maternity rates varied across countries, ranging from 12.2 per 1000 maternities in Italy to 19.4 per 1000 in the Netherlands (Table 1). Triplet maternity rates were much lower than twin maternity rates and the patterns differed from those observed for twins. There were also wide variations in the proportions of multiple births which were preterm (Table 2). The proportion of births before 37 weeks ranged from 68.4% in Austria to 42.2% in Republic of Ireland. The proportion in Republic of Ireland was 62% lower than in Austria and 32% lower than in Flanders. The proportion of births before 32 weeks was also very high, 12.7%, in Austria but there was less variation elsewhere. Variations between countries were fairly similar to the overall proportions of preterm births. Figure 1 compares the distributions of multiple live births by gestational age in six countries in order to see whether observed differences in the preterm birth rates between countries are similar at all gestational ages. For this analysis, two contrasting groups of countries were selected: Austria, Flanders and Germany had the highest preterm birth rates, while Denmark, the Republic of Ireland and Sweden had the lowest rates. Between 33 and 36 weeks, the differences between the two groups of countries gradually widen. After 36 weeks, the gestational age distributions become more variable. The minimum RR of preterm live birth for multiples compared with singletons was 8.2, in Austria (Table 3). The highest
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Table 1. Multiple maternity rates in European countries in 2000* Rate per 1000 total maternities** Country
Number of maternities
Twin births
Triplet and higher order births
All multiple births
Austria Belgium (Flanders) Denmark Finland France Germany (9 Bundesla¨nder) Ireland Italy Netherlands Sweden UK (England and Wales) UK (Northern Ireland) UK (Scotland)
74 559 60 987 65 996 55 852 766 421 549 449 53 549 527 216 197 726 88 331 598 580 21 281 52 598
14.95 18.04 18.32 15.88 14.98 15.82 13.00 11.73 18.98 15.99 14.24 14.75 14.01
0.42 0.30 0.44 0.16 0.28 0.62 0.52 0.52 0.38 0.20 0.44 0.23 0.36
15.37 18.33 18.75 16.04 15.26 16.44 13.52 12.25 19.37 16.19 14.68 14.99 14.37
*2001 in Austria, 1999 in Republic of Ireland and the Netherlands and 1998 in Italy. **Pregnancies leading to one, two or more registrable births.
RR was 10.5, in Finland. At least 17.6% of preterm live births were attributable to multiples in the countries studied. The PAR was above 20% in half of the countries, reaching 24.8 in Denmark. There was a significant correlation between the proportion of preterm births (
