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Spontaneous preterm delivery in primiparous women at low risk in Denmark: population based study

BMJ 2006; 332 doi: (Published 20 April 2006) Cite this as: BMJ 2006;332:937
  1. Jens Langhoff-Roos, consultant (jlr{at},
  2. Ulrik Kesmodel, associate professor2,
  3. Bo Jacobsson, associate professor3,
  4. Steen Rasmussen, statistician4,
  5. Ida Vogel, associate professor5
  1. 1 Juliane Marie Centre, Department of Obstetrics, Rigshospitalet, DK-2100 Copenhagen, Denmark
  2. 2 NANEA at the Institute of Public Health, Department of Epidemiology, University of Aarhus, DK-8000, Denmark
  3. 3 Perinatal Centre, Department of Obstetrics and Gynecology, Sahlgrenska University Hospital/Ostra, SE-41345 Gothenburg, Sweden
  4. 4 National Board of Health, Health Statistics, DK-2300 Copenhagen
  5. 5 Department of Clinical Genetics, Aarhus University Hospital
  1. Correspondence to: J Langhoff-Roos
  • Accepted 30 January 2006


Objectives To analyse trends in preterm delivery and the factors contributing to preterm delivery in Denmark. To construct a standard population at low risk (white European, 20-40 years of age, with a singleton spontaneous pregnancy) and describe the changes in this population so that time trends can be compared internationally.

Design Population based study.

Participants 99.8% of all deliveries in Denmark, 1995-2004.

Main outcome measures Proportion of babies born at less than 37 weeks' completed gestation for each year in the overall population and in a standard population at low risk.

Results Overall, the proportion of preterm deliveries increased by 22% from 1995 to 2004. During the same period, known risk factors for preterm delivery such as in vitro fertilisation, multiple pregnancies, and elective deliveries also increased, and logistic regression analyses showed that these factors were associated with an increased risk of preterm delivery. Spontaneous preterm deliveries in primiparous women at low risk rose 51% (from 3.8% to 5.7%) during this time compared with 20% (2.7% to 3.2%) in multiparous women at low risk.

Conclusions The overall proportion of preterm deliveries increased significantly from 1995 to 2004 and primiparity and multiple birth were the most important contributing factors. The rise in spontaneous preterm deliveries in the standard population of primiparous women at low risk was greater than in the total population.


  • Contributors JL-R conceived the study in collaboration with UK. SR retrieved register data and performed the initial analyses. UK performed additional statistical analyses. All authors contributed to the data interpretation. JL-R wrote the first draft of the manuscript and all authors contributed to the revision.

  • Funding FIGO foundation of the Danish Society of Obstetrics and Gynaecology.

  • Competing interests None declared.

  • Ethical approval Danish Data Protection Agency.

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