Factors that influence the timing of spontaneous labor at term

Whether pre-term birth culminates as a result of a de novo pathologic process or is more simply early activation of physiologic mechanisms is unknown. Exploration of the onset of labor in term women with classical risk factors for early delivery might provide insights into the mechanisms leading to pre-term birth. This study examines whether sociodemographic factors known to increase the risk of pre-term birth also affect the length of term gestations.

From a large prospective cohort composed of women delivering from 1995-2000, a sample was selected of 441 women from Central North Carolina, US, who delivered singletons after 37 weeks gestation. An algorithm was designed to identify induced labors and gestational age was censored at the time of induction. Gestational age was assigned by sonography and menstrual dating. Data were analysed using the Cox proportional hazards model. The main outcome measure was time to spontaneous labor.

Women with 12 years of education had longer periods of gestation than women with less than 12 years of education, HR = 0.57 [0.39, 0.84]. Shorter gestational periods were found for women with pre-term premature rupture of membranes (PPROM) in a previous pregnancy, HR = 3.70 [1.60, 8.52], even after adjusting for confounders. Smoking was not associated (p > 0.1) with the timing of labor at term.

By studying the timing of spontaneous parturition at term we identified that there is little overlap in risk factors that affect timing of delivery between spontaneous term and pre-term births.