Objective To describe the time of onset of contractions which result in preterm delivery to investigate if there is a diurnal influence.
Design The admission register for the neonatal unit was used to identify admissions for prematurity over a three-year period. Obstetric case records were then reviewed to obtain the recorded time of onset of contractions when delivery was preceded by spontaneous labour.
Setting A maternity hospital in the United Kingdom.
Main outcome measures The time of onset of spontaneous contractions which result in preterm labour and delivery.
Results Four hundred and twenty-five women in preterm labour were studied. A significant diurnal rhythm in the timing of onset of contractions was noted with 42% of deliveries occurring in labour which commenced between midnight and 0600 hours. Subgroup analysis indicated that there was a significant rhythm in second trimester preterm labours, male and female babies and that this rhythm was present during both the winter and summer months.
Conclusion The periodicity of preterm labour onset demonstrates a rhythm which is similar to the rhythm in the onset of labour at term. Preterm labour most commonly begins between midnight and 0600 hours.