title: Early versus Late Admission to Labor Affects Labor Progression and Risk of Cesarean Section in Nulliparous Women.
abstract: BACKGROUND
  Rates of cesarean section increase worldwide, and the components of this increase are partially unknown. A strong role is prescribed to dystocia, and at the same time, the diagnosis of dystocia is highly subjective. Previous studies indicated that risk of cesarean is higher when women are admitted to the hospital early in the labor.
  
  
  METHODS
  We examined data on 1,202 nulliparous women with singleton, vertex pregnancies and spontaneous labor onset. We selected three groups based on cervical dilatation at admission: early (0.5-1.5 cm, N = 178), intermediate (2.5-3.5 cm, N = 320), and late (4.5-5.5 cm, N = 175). The Kaplan-Meier estimator was used to analyze the risk of delivery by cesarean section at a given dilatation, and thin-plate spline regression with a binary outcome (R library gam) to assess the form of the associations between the cesarean section in either the first or second stage versus vaginal delivery and dilatation at admission.
  
  
  RESULTS
  Women who were admitted to labor early had a higher risk of delivery by cesarean section (18 versus 4% in the late admission group), while the risk of instrumental delivery did not differ (24 versus 24%). Before 4 cm dilatation, the earlier a woman was admitted to labor, the higher was her risk of delivery by cesarean section. After 4 cm dilatation, however, the relationship disappeared. These patterns were true for both first and second stage cesarean deliveries. Oxytocin use was associated with a higher risk of cesarean section only in the middle group (2.5-3.5 cm dilatation at admission).
  
  
  CONCLUSIONS
  Early admission to labor was associated with a significantly higher risk of delivery by cesarean section during the first and second stages. Differential effects of oxytocin augmentation depending on dilation at admission may suggest that admission at the early stage of labor is an indicator rather than a risk factor itself, but admission at the intermediate stage (2.5-3.5 cm) becomes a risk factor itself. Further research is needed to study this hypothesis.
authors.affiliation: Department for Epidemiology of Infectious Diseases, Hannover Medical School, Hannover, Germany; Department of Epidemiology, Helmholtz-Centre for Infection Research, Braunschweig, Germany.
authors.forename: Rafael T
authors.initials: RT
authors.lastname: Mikolajczyk
authors.affiliation: Shanghai Key Laboratory of Children's Environmental Health, Xinhua Hospital, Shanghai Jiaotong University School of Medicine , Shanghai , China.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Epidemiology Branch, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health , Bethesda, MD , USA.
authors.forename: Jagteshwar
authors.initials: J
authors.lastname: Grewal
authors.affiliation: Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Naval Hospital Camp Lejeune , Camp Lejeune, NC , USA.
authors.forename: Linda C
authors.initials: LC
authors.lastname: Chan
authors.affiliation: Midwifery Research and Education Unit, Department of Gynecology and Obstetrics, Hannover Medical School , Hannover , Germany.
authors.forename: Antje
authors.initials: A
authors.lastname: Petersen
authors.affiliation: Midwifery Research and Education Unit, Department of Gynecology and Obstetrics, Hannover Medical School , Hannover , Germany.
authors.forename: Mechthild M
authors.initials: MM
authors.lastname: Gross
keywords: Kaplan-Meier life table analysis; cesarean section; dilation; early admission; oxytocin; timing of interventions