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Several studies have reported increasing trends in preterm birth in developed countries, mainly attributable to an increase in medically indicated preterm births. Our aim was to describe trends in preterm birth among singleton and multiple pregnancies in the Netherlands.

Design: Prospective cohort study
Setting: Nationwide study
Population: We studied 1 451 246 pregnant women from 2000 to 2007.

We assessed trends in preterm birth. We subdivided preterm birth into spontaneous preterm birth after premature prelabour rupture of membranes (pPROM), medically indicated preterm birth and spontaneous preterm birth without pPROM. We performed analyses separately for singletons and multiples.

Main outcome measures: The primary outcome was preterm birth, defined as birth before 37 weeks of gestation, with very preterm birth (<32 weeks of gestation) being a secondary outcome.

The risk of preterm birth was 7.7% and the risk of very preterm birth was 1.3%. In singleton pregnancies, the preterm birth risk decreased significantly from 6.4% to 6.0% (P < 0.0001), mainly as a result of the decrease in spontaneous preterm birth without pPROM (3.6-3.1%, P < 0.0001). In multiple pregnancies, the preterm birth risk increased significantly (47.3-47.7%, P = 0.047), mainly as a result of medically indicated preterm birth, which increased from 15.0% to 17.9% (P < 0.0001).In the Netherlands, the preterm birth risk in singleton pregnancies decreased significantly over the years. The trend of increasing preterm birth risk reported in other countries was only observed in (medically indicated) preterm birth in multiple pregnancies.