The influence of fetal head circumference on labor outcome: a population-based register study

Objective. To investigate the association between postnatal head circumference and the occurrence of the three main indications for instrumental delivery, namely prolonged labor, signs of fetal distress and maternal distress. We also studied the association between postnatal fetal head circumference and the use of vacuum extraction and emergency cesarean section. Design. Population-based register study. Setting. Nationwide study in Sweden. Population. A total of 265 456 singleton neonates born to nulliparous women at term between 1999 and 2008 in Sweden. Methods. Register study with data from the Swedish Medical Birth Register. Main outcome measures. Prolonged labor, signs of fetal distress, maternal distress, use of vacuum extraction and emergency cesarean section. Results. The prevalence of each outcome increased gradually as the head circumference increased. Compared with women giving birth to a neonate with average size head circumference (35 cm), women giving birth to an infant with a very large head circumference (3941 cm) had significantly higher odds of being diagnosed with prolonged labor [odds ratio (OR) 1.49, 95% confidence interval (CI) 1.331.67], signs of fetal distress (OR 1.73, 95% CI 1.492.03) and maternal distress (OR 2.40, 95% CI 1.962.95). The odds ratios for vacuum extraction and cesarean section were thereby elevated to 3.47 (95% CI 3.103.88) and 1.22 (95% CI 1.041.42), respectively. The attributable risk proportion percentages associated with vacuum extraction and cesarean section were 46 and 39%, respectively among the cases exposed to a head circumference of 3741 cm. Conclusions. Large fetal head circumference is associated with complicated labor and is etiological to a considerable proportion of assisted vaginal births and emergency cesarean sections.