INTRODUCTION: Very few studies describe the obstetric and neonatal outcome of spontaneous pregnancies in women with irregular menstrual cycles. However, menstrual cycle irregularities are common and may be associated with increased risk, and women who develop pregnancy complications more frequently recollect irregular menstrual cycles before the time of conception in case-control studies.

MATERIAL AND METHODS: This retrospective cohort study compares obstetric and neonatal outcomes in spontaneous singleton pregnancies in 3440 primiparous Danish women stratified according to menstrual cycle regularity. All pregnancies delivered after 22 weeks of gestation and had a nuchal translucency examination at Copenhagen University Hospital Hvidovre between 1 January 2009 and 31 December 2010. Menstrual cycle irregularity was defined as more than 7 days' deviation between self-reported and ultrasound examination-based gestational age. Outcome measures were gestational diabetes, hypertension, preeclampsia, preterm premature rupture of membranes, preterm birth, prolonged pregnancy, birthweight, umbilical artery pH <7.1, APGAR <7 after 5 min, admission to neonatal intensive care unit and stillbirth. Women with more than 7 days' deviation between self-reported and ultrasound examination-based gestational age were compared with women with a deviation of 7 days or less.

RESULTS: Irregular menstrual cycle before conception increases the risk of preeclampsia (7.9% vs. 5.2%, p < 0.05) and low birthweight (6.0% vs. 3.6%, p < 0.05) in spontaneous pregnancies, but reduces the risk of prolonged pregnancy (1.4% vs. 4.7%, p < 0.001).

CONCLUSION: Irregular menstrual cycle before conception is associated with increased risk of adverse obstetric and neonatal outcome.