The incidence of preterm deliveries decreases in Finland

Authors


Dr M Jakobsson, Department of Obstetrics and Gynaecology, University Hospital, PO Box 140, FIN-00029, Helsinki, Finland. Email maija.jakobsson@fimnet.fi

Abstract

Objective  We examined the trends and risk factors of preterm delivery.

Design  Register-based retrospective cohort study from Finland.

Setting  National Medical Birth Register data during 1987–2005.

Population  The study population consisted of 1 137 515 deliveries, of which 59 025 were preterm (5.2%).

Methods  We calculated the population attributable risks for using the risk factor prevalence rates in the population. We further calculated odds ratios with 95% CI by multivariate logistic regression to adjust for confounders.

Main outcome measures  Preterm delivery rate subclassified into moderately preterm (32–36 weeks), very preterm (28–31 weeks) and extremely preterm (less than 28 weeks).

Results  Preterm delivery rates increased from 5.1% in the late 1980s to 5.4% in the late 1990s but then decreased to 5.2% for 2001–05. The proportion of extremely preterm deliveries decreased substantially by 12% (P < 0.01). The greatest risk factors were multiplicity (OR 13.72, 95% CI 13.26–14.19), followed by elective delivery (OR 1.86, 95% CI 1.82–1.89), primiparity (OR 1.47, 95% CI 1.45–1.50), in vitro fertilisation treatment (OR 1.39, 95% CI 1.31–1.47), maternal smoking (OR 1.31, 95% CI 1.29–1.34) and advanced maternal age (OR 1.02, 95% CI 1.02–1.03 for each additional year of age). Prematurity rates decreased by 1.8% after adjusting for risk variables.

Conclusions  The rate of preterm delivery has not increased from 1987 to 2005 in Finland, while the risk for extremely preterm delivery has decreased. This finding is in contrast with recent trends in other countries.

Introduction

Preterm delivery is a major cause of perinatal mortality and infant morbidity. The potential long-term consequences are often associated with increased morbidity, also in later life.1 Preterm delivery is a marker for adverse health outcomes. Women having premature deliveries not only have problems in their subsequent pregnancies and deliveries2 but also have an increased risk for cardiovascular disease.3,4

Since 1990, the US preterm delivery rate rose by 20% to 12.7 in 2005.5 The rates of preterm delivery have also been rising in Europe,6 Langhoff-Roos et al.7 recently reported that the proportion of preterm deliveries increased by 22% from 1995 to 2005, and the incidence of spontaneous preterm deliveries among primiparous women at low risk increased most. An Australian study by Tracy et al.8 also showed an increasing trend. If this increasing trend is real and also observed in other countries, the societal and health impacts will be considerable.

We had an opportunity to use high-quality population-based register data to assess risk factors associated with preterm delivery and to investigate the trend for prematurity rates in Finland.

Methods

The study material included information from the Finnish Medical Birth Register (MBR) maintained by STAKES (the National Research and Development Centre for Welfare and Health). The MBR collects baseline data on care and interventions for the mother during pregnancy and delivery and on the newborn’s outcome during the first 7 days. The MBR data are complied at the time of birth, using the mother’s prenatal charts as one of the information sources. Data are collected from all delivery units in Finland. Less than 0.1% of all newborns are missing from the MBR,9 and information on those cases is routinely obtained from the Central Population Register and the Cause-of-Death Register. Following these linkages, the MBR is complete. Data are checked at STAKES, and seemingly incorrect information is returned for revision. For all variables used in this study, the data correspond well with information in hospital records.10

The main outcome of the study was preterm delivery (duration of pregnancy less than 37 weeks), subclassified further into moderately preterm (32–36 weeks), very preterm (28–31 weeks) and extremely preterm delivery (less than 28 weeks).

We used the MBR data from 1987 to 2005 with information on 1 137 515 deliveries, of which only 8135 (0.7%) lacked information on gestational age. The following variables were used to study the risk factors of preterm delivery: mode of conception (spontaneous, in vitro fertilisation [IVF]), mode of induction of delivery (spontaneous, elective consisting of elective caesarean sections and elective inductions of labour), parity (multiparous, primiparous), multiplicity (singleton, multiple), maternal age (continuous) and smoking (nonsmoker, smoker).

The total duration of the study was divided into four periods: 1987–90 (period 1), 1991–95 (period 2), 1996–2000 (period 3) and 2001–05 (period 4).

Information on IVF deliveries was only available from 1991 onwards. However, only 170 IVF children were born in Finland before 1991 (0.3‰ of all deliveries), and these were not included in our data. All other variables were available for the whole study period. For maternal smoking, our data after 1991 also included information on those parturients who stopped smoking during the first trimester.

To limit the analysis to the low-risk population, we selected a standard population consisting of primiparous nonsmoking women aged 20–40 years with a singleton spontaneous pregnancy and spontaneous delivery.

We calculated the population attributable risks for using the risk factor prevalence rates in the population. We further calculated odds ratios with 95% CI by multivariate logistic regression to adjust for confounders. These regression analyses were performed separately for extremely preterm, very preterm, moderately preterm and all preterm deliveries and for each time period.

STAKES, as the register keeper, gave the necessary authorisation required by national data protection legislation.

Results

The study population consisted of 1 137 515 deliveries during 1987–2005, of which 59 025 were preterm (5.19%) and 1 070 355 (94.77%) were term deliveries (Table 1). Of preterm deliveries, 4140 (0.37%) were extremely preterm, 5912 (0.52%) very preterm and 48 973 (4.34%) moderately preterm.

Table 1.  Term and preterm deliveries in Finland from 1987 to 2005
 Period 1 (1987–90), n (%)Period 2 (1991–95), n (%)Period 3 (1996–2000), n (%)Period 4 (2001–05), n (%)Total, n (%)
  • *

    <37 gestational weeks.

  • **

    <28 gestational weeks.

  • ***

    28–31 gestational weeks.

  • ****

    32–37 gestational weeks.

  • *****

    20- to 40-year-old primiparous women with a singleton spontaneous pregnancy.

All preterm*12 673 (5.07)16 312 (5.08)15 464 (5.39)14 576 (5.22)59 025 (5.19)
Extremely preterm**964 (0.39)1195 (0.37)1023 (0.36)958 (0.34)4140 (0.37)
Very preterm***1289 (0.52)1608 (0.50)1571 (0.55)1444 (0.52)5912 (0.52)
Moderately preterm****10 420 (4.22)13 509 (4.24)12 870 (4.51)12 174 (4.37)48 973 (4.34)
Term234 127 (94.87)302 646 (94.89)269 632 (94.58)263 950 (94.77)1 070 355 (94.77)
No information3274232515999378135
Births total250 074321 283286 695279 4631 137 515
Low-risk population*****82 667 (33.1)104 468 (32.5)90 902 (31.7)85 953 (30.8)363 990 (32.0)

The number of all preterm deliveries first increased from 5.07% in period 1 to 5.39% in period 3 (P < 0.001) and then decreased to 5.22% in period 4 (P < 0.01). The trend was similar in the moderately preterm group, that is first, an upward trend from 4.22% in period 1 to 4.51% in period 3 (P < 0.001) and then a decrease to 4.37% in period 4 (P < 0.05). The changes in the very premature group were not significant, but the proportion of extremely preterm deliveries decreased by 11.9% from 0.39% in period 1 to 0.34% in period 4 (P < 0.01). The trends were similar when multiple deliveries were excluded.

The rates of most known risk factors for preterm delivery increased during the study period (Table 2). Primiparity increased from 39 to 42% (P < 0.001). Elective deliveries, that is elective caesarean sections, and inductions of labour increased from 17 to 21% (P < 0.001). The population of IVF deliveries more than doubled between 1990 and 2005, from 0.7% of all deliveries to 1.9% (P < 0.001). The number of multiple births first increased from 1.1% in period 1 to 1.6% in period 3 (P < 0.01) but decreased to 1.5% in period 4 (P < 0.001). Maternal smoking was quite stable, approximately 15% of all parturients. However, the proportion of women who smoked throughout pregnancy decreased from 14% in period 2 to 12% in period 4 (P < 0.001).

Table 2.  Risk factors for prematurity in Finland from 1987 to 2005
 Period 1 (1987–90), n (%)Period 2 (1991–95), n (%)Period 3 (1996–2000), n (%)Period 4 (2001–05), n (%)Change % (period 1–4)Change % (period 2–4)Change % (period 3–4)
  1. NA, not available.

IVFNA2256 (0.7)5680 (2.0)5243 (1.9)NA167.2−5.3
Multiple births2852 (1.1)4476 (1.4)4716 (1.6)4244 (1.5)33.29.0−7.7
Primiparity98 762 (39.4)128 453 (40.0)115 760 (40.4)117 560 (42.1)6.55.24.2
Elective delivery42 485 (17.0)63 592 (19.8)63 749 (22.2)57 646 (20.6)21.44.2−7.2
Smoking37 647 (15.1)50 855 (15.8)41 776 (14.6)41 603 (14.9)−1.1−5.92.2
Smoking through pregnancyNA45 692 (14.22)37 511 (13.08)34 050 (12.18)NA−14.3−6.9

After adjusting for the risk factors, the proportion of preterm deliveries decreased by 1.8% (95% CI 1.0–2.6%) each period for all subgroups. The rate of extremely preterm deliveries decreased most, by 7.3% (95% CI 4.6–9.9%). For very preterm deliveries, the decrease was 3.3% (95% CI 0.9–5.6%), while the decrease was smallest for moderately preterm deliveries, that is 1.0% (95% CI 0.1–1.8%).

As expected, multiplicity was the greatest risk factor for prematurity (OR 13.72, 95% CI 13.26–14.19) (Table 3). The risk for being premature was highest in the moderately preterm group (OR 11.95, 95% CI 11.53–12.38), but the risk was eight- to nine-fold even in other preterm groups. Elective delivery was the second most important risk factor with an odds ratio of 1.86 (95% CI 1.82–1.89), and this risk was at its highest in the very premature group (OR 3.26, 95% CI 3.10–3.44). We also found a risk for being premature among primiparas (OR 1.47, 95% CI 1.45–1.50), and the risk was similar for very preterm and moderately preterm deliveries. The risk for prematurity was also associated with IVF treatment (OR 1.39, 95% CI 1.31–1.47), and this risk was the highest in the moderately preterm group. Also smoking increased prematurity (OR 1.31, 95% CI 1.29–1.34), and its effect was the greatest for extremely preterm and moderately preterm deliveries. Maternal age was another risk factor with an odds ratio of 1.02 (95% CI 1.02–1.03), indicating a 2% increased risk for each additional year in age. Results were similar when multiple births were excluded.

Table 3.  Multivariate logistic analysis of risk factors associated with preterm delivery in Finland from 1987 to 2005
 Extremely preterm, OR (95% CI)Very preterm, OR (95% CI)Moderately preterm, OR (95% CI)All preterm, OR (95% CI)Prematurity (%)
  • NA, not available.

  • *

    Odds ratio for one-year change in age.

  • **

    Ratio for one period.

Multiplicity: multiple vs singleton8.10 (7.33–8.96)8.15 (7.51–8.84)11.95 (11.53–12.38)13.72 (13.26–14.19)44.4
Mode of delivery: elective vs spontaneous1.88 (1.76–2.00)3.26 (3.10–3.44)1.67 (1.63–1.70)1.86 (1.82–1.89)17.2
Parity: multiparous vs primiparous1.27 (1.07–1.52)1.49 (1.41–1.57)1.48 (1.45–1.51)1.47 (1.45–1.50)6.2
Mode of conception: IVF vs spontaneous1.27 (1.07–1.52)1.18 (1.02–1.36)1.34 (1.27–1.42)1.39 (1.31–1.47)17.1
Smoking: smoker vs nonsmoker1.43 (1.32–1.54)1.47 (1.38–1.57)1.27 (1.24–1.30)1.31 (1.29–1.34)6.3
Maternal age*1.04 (1.03–1.04)1.03 (1.02–1.03)1.02 (1.02–1.02)1.02 (1.02–1.03)NA
Period**0.93 (0.90–0.95)0.97 (0.94–0.99)0.99 (0.98–1.00)0.98 (0.97–0.99)NA

When analysed by period, the relative risks (RRs) remained stable. However, the RR for being premature for multiple births and primiparity increased from period 1 to period 4. By time, elective delivery was associated with decreasing risk of prematurity. The same was true for smoking (data not shown).

We also calculated population attributable risks for the key risk factors. These were 17.2% for elective delivery, 13.8% for primiparity, 11.0% for multiplicity, 3.8% for IVF and 3.4% for smoking.

We studied separately the standard low-risk parturients, the proportion of which decreased from 33 to 30% during the study period. These women had 13 250 preterm deliveries (3.64% of all deliveries): 905 extremely preterm (0.25%), 1269 very preterm (0.35%) and 11 076 moderately preterm deliveries (3.04%). These low-risk parturients had a decreased risk for prematurity (OR 0.63, 95% CI 0.62–0.64) compared with other parturients. The risk was smallest in period 3, which reflects the excess number of multiple births after IVF treatments during this period.

Discussion

The overall proportion of preterm deliveries increased from 5.1% in the late 1980s to 5.4% in the late 1990s but then decreased to 5.2% for 2001–05. Most striking was that extremely preterm deliveries decreased substantially between 1987 and 2005. These results are the opposite of trends reported from other Nordic countries, Australia and the USA.7,8,11,12

This study was based on national population-based register data, which rule out sampling error. Finland has an excellent health information system mainly based on register data, which are of high quality and good coverage.9,10 The absence of reporting, recall or participation biases increases the credibility of our results. However, the number of available demographical and epidemiological variables included in the register data is limited, which restricts its utilisation.

Determination of gestational age by ultrasound may increase preterm delivery rates.13 Ultrasound examination was integrated into clinical practice already in the beginning of our study period. Since early 1990s, the MBR has collected information on ultrasound examinations during pregnancy and 85% of all parturients had at least one such examination in 1992. The MBR has constantly based the gestational age data on the best clinical estimate, and therefore, changes in definitions and ascertainment methods should have no impact on our findings. Underreporting of preterm deliveries may be a problem among women of low socio-economic status, who are at the greatest risk for preterm delivery.14 However, the number of cases with no information on gestational age in our study was very low (0.7%).

At least three factors may contribute to the favourable trend in Finland. First, a decreasing trend in preterm deliveries is related to the progress in IVF treatments, that is multiple births after IVF can be prevented by transferring only one embryo. In Finland, the proportion of multiple births is declining and low (15/1000 births in 2005),15 after introducing single embryo transfers into clinical practice.15,16,17 It is noteworthy that Finland has achieved this change through the own initiative of the IVF-clinics.

Second, iatrogenic preterm deliveries have decreased since elective deliveries in early gestational weeks have decreased. Furthermore, in contrast to many other countries, the proportion of caesarean sections has remained quite stable, that is 15.6% in 1994 and 16.8% in 2005.12 For instance, in the USA, the increased preterm delivery rate largely resulted from an increase in induced deliveries,11 but in Finland, this has become less common.

The prevalence of overweight and obesity has increased dramatically in Western industrialised countries including Finland. The proportion of overweight or obese women aged 25–44 years with a body mass index (BMI) of 25 kg/m2 or more has increased from 23% in the late 1970s to 38% in 2006.18 The MBR has complete information on maternal height and pre-pregnancy weight since 2006. In that year, the prematurity rate was 5.6% among parturients with normal weight (BMI 20–24.9 kg/m2) but 6.8% among obese parturients (RR 1.21, 95% CI 1.08–1.34). During the whole study period, the data on overweight and obesity were not available. However, the declining prematurity rates conflict with the increasing overweight and obesity trends, and thus, our results cannot be overvalued.

Third, we demonstrated an increasing trend of smoking cessation during pregnancy. Smoking is a known risk factor for preterm delivery.2 Our findings confirm that it is important to encourage pregnant women to stop smoking.

Langhoff-Roos et al. from Denmark reported a higher increase of prematurity for primiparous low-risk women. The explanation remained unclear.7 Strikingly, our results were the opposite; the risk of prematurity for standard parturients decreased. In general, Finland and Denmark share many common features. Both are Nordic welfare countries. The general fertility rate is at the same level in Finland and Denmark. The number of births outside marriage is high in both countries (39% in Finland and 45% in Denmark in 2000), although most women are cohabiting. For maternal age and smoking, the differences between Finland and Denmark are minor.19 It is unlikely that these factors explain the observed difference in preterm delivery trends between Finland and Denmark. However, the proportion of multiple deliveries seems to increase in Denmark because of extensive use of fertility treatments and multiple embryo transfers.12,16 Finnish women have a higher education level than Danish women, which may in part explain the lower prematurity rate (C. Bjørk et al., unpubl obs.).

There are substantial differences in prematurity rates by ethnic origin and race. It is well known that black women have more preterm deliveries than white women.2,11,20,21 The Finnish MBR does not include information on ethnicity. According to a Finnish study from 1999 to 2001, the preterm delivery rate was lower among Western migrants than among Finns, confirming the healthy migrant effect.22,23 Non-Western migrants, however, had higher preterm delivery rate than the parturients of Finnish origin.24 Their population share is only 2%, which gives a low population attributable risk and cannot explain the difference. In general, Denmark has more migrants than Finland, which may partly explain the differences between these countries. However, Langhoff-Roos et al.7 reported an increased prematurity rate also among standard primiparous women, excluding migrants. Differences in macroeconomic development may also explain the different trends. In the beginning of the 1990s, Finland faced a severe recession followed by a long period of steady economic growth, while Denmark had a recession in the 1980s, followed by favourable economic development thereafter. In our study, the incidence of preterm deliveries was the highest in the late 1990s, which suggests that the recession had only a small or no effect on preterm delivery rates in Finland (C. Bjørk et al., unpubl obs.). There is no consensus on how the recession affected socio-economic differences and prematurity in Finland.25

Danish researchers proposed that increased psychological stress among women who try to cope with work and family is one explanation.7 A characteristic feature of all Nordic countries is the high rate of working women. More women in Finland are working full-time than in other Nordic countries.19 Thus, our results do not support the theory that stress explains prematurity.

In conclusion, we found that the incidence of preterm delivery in Finland has not increased, and the rates are even decreasing after adjustment for all risk factors. Most striking was the decrease in extremely preterm deliveries. Most preterm deliveries follow spontaneous labour, and the causality is poorly understood. Prevention of preterm delivery remains elusive, but some actions can be taken. Iatrogenic preterm delivery should be avoided if possible. The number of transferred embryos should be reduced to avoid multiplicity in IVF. Pregnant women should also be encouraged to stop smoking.

Contribution to authorship

M.J. conceived the study in collaboration with A-M.T., M.G. and J.P. M.G. retrieved register data and performed the statistical analyses. All authors contributed to the data interpretation. M.J. wrote the first draft of the manuscript and all authors contributed to the revision and accepted the final version.

Details of ethics approval

The analyses were performed after the register keeping organisation, STAKES, had given the authorisation required by the national data protection legislation.

Acknowledgements

We thank Medical writer, Mark Philips, language reviser at STAKES.

Ancillary