Zeitlin Et Al-2007-BJOG- An International Journal of Obstetrics & Gynaecology

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Variation in rates of postterm birth in Europe:reality or artefact?J Zeitlin,a B Blondel,a S Alexander,c G Bréartb and the PERISTAT Group

a INSERM, UMR S149, Epidemiological Research Unit on Perinatal and Women’s Health, Paris, France, b Hôpital Tenon, Université Pierre et Marie

Curie-Paris 6, Paris, France, c Reproductive Health Unit, School of Public Health, Université Libre de Bruxelles, Brussels, Belgium

Correspondence: Dr J Zeitlin, INSERM U149, Site St Vincent de Paul, 82 av. Denfert Rochereau, 75014 Paris, France.

Email [email protected]

Accepted 5 February 2007. Published OnlineEarly 6 July 2007.

Objective To compare rates of postterm birth in Europe.

Design Analysis of data from vital statistics, birth registers, and

national birth samples collected for the PERISTAT project.

Setting Thirteen European countries.

Population All live births or representative samples of births for

the year 2000 or most recent year available.

Methods Comparison of national and regional rates of postterm

birth. Other indicators (birthweight, deliveries with

a non-spontaneous onset and mortality) were used to assess the

validity of postterm rates.

Main outcome measures The proportion of births at 42

completed weeks of gestation or later.

Results Postterm rates varied greatly, from 0.4% (Austria,

Belgium) to over 7% (Denmark, Sweden) of births. Higher

postterm rates were associated with a greater proportionof babies with birthweight 4500 g or more. Fetal and early neonatal

mortality rates were higher among postterm births than

among births at 40 weeks. Countries with higher proportions of

births with a nonspontaneous onset of labour had lower postterm

birth rates. The shapes of the gestational-age distributions

at term varied. In some countries, there was a sharp cutoff in

deliveries at 40 weeks, while elsewhere this occurred at

41 weeks.

Conclusions These results suggest that practices for managing

pregnancies continuing beyond term differ in Europe and raise

questions about the health and other impacts in countries with

markedly high or low postterm rates. Some variability in these

rates may also be due to methods for determining gestational age,

which has broader implications for international comparisons of

gestational age, including rates of postterm and preterm births and

small-for-gestational-age newborns.

Keywords Gestational age distribution, perinatal health indicators,

postterm births.

Please cite this paper as: Zeitlin J, Blondel B, Alexander S, Bréart G and the PERISTAT Group. Variation in rates of postterm birth in Europe: reality or artefact?

BJOG 2007;114:1097–1103.

Introduction

Babies born postterm, defined as a gestational age of 42 com-

pleted weeks and over, are at higher risk of poor perinatal

outcome.1 Accordingly postterm birth rates are commonly

proposed as an indicator for monitoring perinatal health.

Management of prolonged pregnancy usually follows one of two general approaches: proposing induction to all pregnant

women before they reach 42 weeks of gestation or close mon-

itoring of pregnancy after 41 weeks with selective induction in

case of fetal distress or a favourable Bishop score. A policy of

systematic induction appears to reduce the caesarean rate and

may be associated with a reduced perinatal mortality rate, but

not all studies are concordant.2,3 Systematically proposing

induction is a long-standing policy in many countries, includ-

ing France and Canada;2,4 in 2004, the American College of

Obstetricians and Gynecologists updated their guidelines to

promote a policy of systematic induction more actively but

recommended that women ‘with unfavourable cervixes can

undergo labour induction or be managed expectantly’.5

Studies from Australia, Canada, and the USA report recent

rates of births after 41 weeks of gestation,6–8 but European dataon these rates are not available. This analysis uses population-

level data on gestational ageto compareratesof births at 42weeks

of gestation or later in European countries; these data were

collected as part of the PERISTAT project on perinatal health

indicators. Our aim is to describe postterm birth rates and to

compare current practices for postterm pregnancies in Europe.

Comparing postterm rates brings up the question of

how gestational age is determined. Using the date of the last

ª 2007 The Authors Journal compilation ª RCOG 2007 BJOG An International Journal of Obstetrics and Gynaecology

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DOI: 10.1111/j.1471-0528.2007.01328.x

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menstrual period (LMP) instead of ultrasound to date preg-

nancies increases the proportion of pregnancies reported as

postterm, in particular because LMP calculations assume that

ovulation occurs 14 days after the first day of the menstrual

period for all women, when actual cycle length varies consid-

erably and is on average longer than 28 days.9–11 We address

this question by using other national-level indicators, includ-

ing birthweight, induction practices, and gestation-specific

mortality to validate reported postterm rates.

Methods

Data come from the European PERISTAT I project, which

aimed to develop a recommended indicator set to describe

and to monitor perinatal health in Europe.12 These indicators

concerned child and maternal health, risk factors, and med-

ical practices. The project also undertook a feasibility study in

the statistical offices and health departments of the parti-

cipating countries to assess whether the recommended in-

dicators could be collected with the definitions proposed. Aquestionnaire was developed that requested data in the form

of numbers of events (births or deaths) for each indicator.

Participants were asked to provide national data for their

country, insofar as possible. Where data were not available

for all parts of a country, but population-based data were

available from one or more regions, these data could be

provided instead. The data used in this analysis came from

sources listed in the Appendix.

For this analysis, we used gestational age data, which were

requested according to week of gestation separately for live

births and stillbirths, as well as for singleton and multiple

births. Multiple births were excluded from the analysis. The

data collection instrument included a question asking how

gestational age was determined. Most providers stated that

it represented the obstetric estimate in the medical records

or that they did not know. The analysis includes data on

gestational-age distribution in Austria, Belgium (both the

Flanders region and the French community), Denmark,

Finland, France (data from the national perinatal survey,

a sample survey of 1 week of births in France), Germany (9

Bundesländer), Ireland, Italy, Luxembourg, the Netherlands,

Portugal, Sweden, and Northern Ireland and Scotland from

the UK. Data from Portugal were not available for multiples

and singletons separately, so all births were included in this

analysis. Portugal also provided data by gestational-agegroups rather than by week of gestation. Data cover the year

2000, except for France and Italy, whose data date from 1998,

the Netherlands (1999), and Austria (2001).

This analysis also considered the following additional in-

dicators: the proportion of live births with a birthweight of

4500 g or more, the proportion of term deliveries with a non-

spontaneous onset (i.e. induction of labour or caesarean sec-

tion before labour), gestational-age-specific fetal mortality,

early neonatal mortality at term and postterm, and the general

distribution of term births by gestational age. Not all countries

could provide information on mode of onset of labour. For

Luxembourg, data on mode of onset were available only for all

births and did not distinguish preterm from term or postterm

births. The gestational-age-specific stillbirth rate was com-

puted as the number of fetal deaths at a given week of gestation

over the number of fetuses in utero at the start of that week.13,14

The gestational-age-specific risk of early neonatal death (


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elsewhere at 41 weeks. The percentage of births at 37 weeks

also varied more, ranging from 5 to 10% in countries with low

postterm rates and from 5 to 7% in the countries with a higher

postterm rate. Finally, the gestational-age curve in Austria

showed a marked shift to the left with a mode at 39 weeks.

Discussion

This analysis documented a large variation in postterm birth

rates in European countries: from a low of 0.4% to a high of

8%. We also found significant correlations between the post-

term rate and other national- and regional-level indicators

that we expected would vary with the postterm rate. There

was a strong correlation between rates of postterm births and

of birthweights over 4500 g, which is consistent with studies

showing that high-birthweight babies are more common

among births after 41 weeks.15 Fetal and early neonatal mor-tality were higher among babies born at 42 weeks and later

compared with those born at 40 weeks in the five countries

with the highest postterm rates, as observed generally in stud-

ies of postterm babies.1 Finally, the proportion of deliveries

with a nonspontaneous onset was also significantly correlated

with the postterm rate in countries that could provide these

data, which shows that lower postterm rates were associated

with medical intervention. These results suggest that Euro-

pean countries differ significantly in their policies and prac-

tices for managing pregnancies that continue past term. This

interpretation is concordant with other studies that have

documented marked differences in the management of preg-

nancy and delivery in Europe.16–19

Before we reach this conclusion, however, we must consider

to what extent differences in the measurement of gestational

age explain the variability between countries. A large literature

shows that the methods used to determine gestational age

influence the postterm rate. Postterm rates based on LMP are

about 3.5 times higher than postterm rates based on ultrasound

measures alone.9,11,20,21 Studies show that when ultrasound is

used to establish gestational age, its distribution shifts to the

left. Ultrasound use also results in fewer errors associated with

poor recall or irregular cycles. Since these errors are propor-

tionally more important at the extremes of the distribution,

their reduction also contributes to a decrease in births recordedas postterm. Randomised trials and observational studies both

report that the use of early ultrasound dating reduces induction

for postterm and the proportion of births that occur at 42

weeks and after,22,23 although one trial found no such differ-

ence.24 Other differences in the way that gestational age is

determined, such as rounding up instead of using completed

weeks25 or the use of different ultrasound curves,26 may also

affect estimates.

Table 1. Pregnancy outcome for singleton live births in European countries participating in the PERISTAT I project

Country Total births Postterm rate

( ‚ 42 weeks of gestational age)

Births with

weight ‚ 4500 g

Term births

with nonspontaneous onset*

% 95% CI % 95% CI % 95% CI

Austria 73 122 0.4 0.4–0.4 1.1 1.1–1.2 n/aBelgium (Flanders) 59 624 0.6 0.5–0.7 1.0 0.9–1.1 41.2 40.8–41.6

Belgium (French community) 42 779 0.4 0.3–0.5 0.6 0.5–0.7 39.7 39.2–40.2

Denmark 64 469 8.1 7.9–8.3 4.3 4.1–4.5 15.0 14.7–15.3

Finland 54 753 4.4 4.2–4.6 3.4 3.2–3.6 21.1 20.8–21.4

France (national survey) 13 133 1.2 1.0–1.4 0.8 0.6–1.0 28.4 27.6–29.2

Germany (9 Bundesla ¨ nder) 538 407 2.3 2.3–2.3 1.7 1.7–1.7 24.1 24.0–24.2

Ireland 52 554 6.7 6.5–6.9 3.1 3.0–3.2 n/a

Italy 520 620 2.7 2.7–2.7 0.7 0.7–0.7 n/a

Luxembourg** 5275 0.9 0.6–1.2 0.8 0.6–1.0 43.4 42.1–44.7

Netherlands 192 621 5.2 5.1–5.3 2.4 2.3–2.5 n/a

Portugal*** 120 071 2.1 2.0–2.2 0.8 0.7–0.9 n/a

Sweden 86 583 7.5 7.3–7.7 4.4 4.3–4.5 15.5 15.3–15.7

UK: Northern Ireland 21 045 1.5 1.3–1.7 2.5 2.3–2.7 46.1 45.7–46.5

UK: Scotland 50 683 3.0 2.9–3.1 2.1 2.0–2.2 34.8 34.2–35.4

Rank correlation with postterm

birth rate (P value)

.76 (.001) 2.78 (.008)

n/a, not available.

*Induced delivery or caesarean section before the onset of labour.

**Induction practices for all births, including preterm births.

***All births, including multiples.

Postterm births in Europe


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There are several reasons to believe that the observed vari-

ation is not solely due to methods for determining gestational

age. First, the use of ultrasound for dating pregnancies is

a routine part of antenatal care in European countries, even

those with high postterm rates. A study of Swedish maternity

units found that all units routinely used ultrasound to date

pregnancies by the early 1990s.27 In Denmark, in a study of

postterm births before 1994,28 almost all the women were

offered ultrasound in cases where LMP was uncertain, and

a 1995 study of ultrasound use found that 93% of all women

had at least one ultrasound scan during pregnancy, and 74%

before 21 weeks.29

Second, even systematic use of ultrasound to date pregnan-

cies would not result in postterm rates close to zero, as we

observed in several countries, unless they also had an active

policy to induce postterm births. Studies comparing ultra-

sound to LMP for pregnancy dating9,11,20,21 showed that use

of ultrasound alone yielded postterm rates from 2 to 3.5% in

North American, British, and Finnish populations. When a

7-day rule is used, i.e. gestational age is adjusted if there is

more than a 7-day discrepancy with the LMP estimate, ratesof postterm vary from 2.5 to 4.5%. In all these observational

studies, the rates of postterm births reflect decisions made by

obstetricians to induce delivery for prolonged pregnancy, as

well as the methods used to determine gestational age.

The comparison of gestational-age distributions provides

further evidence that real differences exist between countries.

Inductions for prolonged pregnancy should lead to an increase

in deliveries at 41 weeks,30 whereas systematic use of ultrasound

leads to an overall shift of the distribution to the left and

a decrease in extreme values. The curve of the gestational-age

distributions in countries with a high postterm rate was bell

shaped and started to decline gradually at 40 weeks. It, thus,differed from the curve in countries with a lower postterm rate,

where there was a clear drop-off at either 40 or 41 weeks.

Nonetheless, the shapes of these distributions also raise ques-

tions about measurement differences: for instance, compared

to other countries, the curve in Austria was shifted to the left, as

might be predicted by consistent use of ultrasound for dating,

while the curve in Ireland had a peak at 40 weeks of gestation,

which might be suggestive of ‘lumping’ at 40 weeks. While all

0,0%

5,0%

10,0%

15,0%

20,0%

25,0%

30,0%

35,0%

40,0%

37 38 39 40 41 42 43

Denmark Finland Ireland Netherlands Sweden

0,0%

5,0%

10,0%

15,0%

20,0%

25,0%

30,0%

35,0%

40,0%

37 38 39 40 41 42 43

Austria Be: Flanders Be: French France UK:N Ireland

(a)

(b)

Figure 1. (a) Gestational-age distribution at term in countries with

a postterm rate of 4% or higher and (b) 1.5% or lower.

Table 2. Stillbirth rates in each week per 1000 undelivered fetuses at the beginning of the week and rate of neonatal deaths per 1000 live births

in countries with a postterm rate of 4% or more*

Week of

gestation

Fetuses undelivered

at beginning

of week

Stillbirths

that week

Stillbirth rate

per 1000

undelivered fetuses

95% CI Live births Early neonatal

deaths

(


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countries may use ultrasound routinely in antenatal care, the

use of information from ultrasound to determine gestational

age may well differ. For example, one factor that may affect use

of ultrasound information is its timing. Obstetricians may be

more willing to make changes to gestational-age estimates after

very early ultrasound. One randomised study from the USA

comparing first versus second trimester ultrasound found that

41% of women randomised to the first trimester group had

their estimated date of confinement (EDC) adjusted; the rule

for adjustment was to change the EDC if the LMP and ultra-

sound estimates differed by 5 days or more. In the group

assigned to a second trimester scan, a 10-day rule was used

and resulted in adjusting 11% of estimates.22 A Swedish study

on practices before the universal adoption of ultrasound found

that 31% of women receiving care in hospitals offering routine

ultrasound dating had their EDC adjusted, with interhospital

differences of 18 to 65%.27 A more recent randomised study,

however, only found that 5.7% of women receiving a scan

between 8 and 12 weeks had their dates readjusted with a

5-day decision rule.24

To our knowledge, there are no com-parative studies on how ultrasound scans are used for dating

pregnancies within or between countries in Europe.

Disentangling the issue of measurement from that of

obstetric practices may not be possible since decisions to

induce delivery or to plan a caesarean section depend on

the accuracy of the gestational-age estimate. An obstetrician

would be more likely to induce delivery at 41 weeks if it is

determined by ultrasound early in pregnancy than if gesta-

tional age is uncertain or if the ultrasound is done later, when

the margin of error is larger. Furthermore, reliance on ultra-

sound alone to date pregnancies may be more common in

more medicalised contexts where induction or planned cae-

sareans are more common anyway.

The differences in postterm rates raise questions about

potential health and other impacts. We found that babies born

at 42 weeks or later in countries with a relatively high postterm

rate had higher fetal and neonatal mortality than babies born

at 40 weeks of gestation. This finding suggests that more active

management of these pregnancies might help lower mortality

in these countries. A study from a region in Norway with

a postterm rate of 7.6% reached a similar conclusion after

documenting excess mortality, lower Apgar scores, and more

neonatal intensive care admissions among postterm babies.31

Studies in the USA and Canada have linked increasing induc-

tion of labour at term to declining fetal mortality rates. 30,32However, an assessment of these practices must include all

pregnancies, as more active policies to induce postterm preg-

nancies tend to be associated with more active intervention

for all pregnancies. Several recent studies report that decreas-

ing postterm rates are accompanied by a shift to the left of the

gestational-age distribution and a concomitant increase in the

proportion of elective caesarean deliveries and inductions of

labour at 38 and 39 weeks as well as late preterm births.6,8 We

also observed a higher proportion of deliveries at 37 weeks in

some countries with low postterm rates than in those with

high postterm rates. A valid health assessment of these prac-

tices must, therefore, include all births. This analysis was

not possible with our country-level data as we could not

control for known demographic, socio-economic, and medical

factors that influence perinatal mortality and which vary

between the countries in this study. Other trade offs, related

to the resources used for induction of deliveries, the potential

complications of inductions, and planned caesareans,2 as well

as women’s preferences about the onset of delivery, should

also be considered in evaluations of these practices. 33,34

Our data come frombirth registers at the regional or national

level and thus provide limited possibilities for pursuing these

questions. We feel that our results underscore the value of these

data, which can provide valuable insights, especially when they

reveal marked variation in outcomes or practices. The analysis

of routine birth or medical registers cannot substitute for spe-

cific epidemiological studies where preexisting protocols avoid

the pitfalls of measurement error and unavailable data on key confounders; however, it does provide a context for assessing

practices and helps to frame questions for future research.

Conclusion

We conclude from these data and our review of the literature

that differences between European countries in the propor-

tion of births after 41 weeks represent more than a measure-

ment artefact. These results raise questions about the impact

of these differences on maternal and child health, resource

use, and women’s experiences of delivery in European coun-

tries. However, we also believe that some of the variability

reflects differences in the measurement of gestational age,although we were unable to assess the magnitude of this effect

in this study. Key perinatal health indicators are based on

gestational age; these include not only the postterm rate but

also the preterm birth rate and the proportion of small-for-

gestational-age births.35 Further research on practices for

determining gestational age and on how these practices affect

the gestational-age distribution is essential for comparisons of

these indicators between countries and perhaps between

regions or even maternity hospitals within countries.

The PERISTAT group

Steering committeeJ Zeitlin, K Wildman, G Bréart, France (project coordinators);

S Alexander, Belgium; H Barros, Portugal; B Blondel, France;

S Buitendijk, the Netherlands; M Gissler, Finland; A Macfar-

lane, UK.

Scientific advisory committeeC Bakoula, Greece; F Bolumar, Spain; J Bottu, Luxembourg;

S Cnattingius, Sweden; M Cuttini, Italy; P Defoort, Belgium;



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J-B Gouyon, France; L Krebs, Denmark; W Künzel, Germany;

N Lack, Germany; M Langer, Austria; J Langhoff-Roos,

Denmark; G Lindmark, Sweden; S Marchant, UK; N Monte-

negro, Portugal; F Morcillo, Spain; M Newburn, UK; JG Nij-

huis, the Netherlands; S Prati, Italy; A Staines, Ireland; M

Virtanen, Finland; N Vitoratos, Greece; C Vutuc, Austria; Y

Wagener, Luxembourg.

Acknowledgement

The PERISTAT study was partially funded by the Directorate

for Health and Consumer Protection (DG-SANCO) of the

European Commission. j

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Appendix. Data sources used for constructing tables

Country Coverage (if not national) Data source* Year

Austria Statistics Austria 2001

Belgium Flanders SPE (Studiecentrum voor

Pernatale Epidemiologie)

2000

Belgium French Community ONE (Office de la Naissance et de l’Enfance) 2000

Denmark Danish perinatal database 2000

Finland Medical birth registry—STAKES 2000

France Representative sample National Perinatal Survey 1998

Germany 9 Bundesländer** BAQ—perinatal survey 2000*

Ireland National Perinatal Reporting System 1999

Italy ISTAT, Civil birth and death registration.

Discontinued in 1998

1998

Luxembourg FIMENA 2000 2000

Netherlands Merged database from professional registers. Landelijke

Verloskunde Registratie (National Perinatal Register):

data on course of pregnancy and delivery. Landelijke

Neonatologie Registratie (National Neonatology Register):

diagnoses of the child, duration of hospital stay, treatments

1999

Portugal Estatisticas Demograficas,Estatisticas de Saude, INE,

Instituto Nacional de Estatistica

1999

Spain National Institute for Statistics (INE) 1999

Sweden Medical Birth Register 2000

UK Scotland Information and Statistics Division, SMR2 Maternity

Discharge Sheet

2000

UK Northern Ireland Perinatal Information, Northern Ireland, aggregated

data from child health systems

2000

*More detail on data sources available in Macfarlane et al.13

**Bayern, Baden-Württemberg, Berlin, Hessen (data from 2001), Niedersachsen and Bremen, Nordrhein, Sachsen, Thüringen, Westfallen-Lippe,

representing 72.6% of all births.



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Zeitlin Et Al-2007-BJOG- An International Journal of Obstetrics & Gynaecology

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