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Background: Purpose of this study was to investigate differences between primiparous term pregnancies, one leading to vaginal delivery (VD) and the other to acute cesarean section (CS) due to labor dystocia in the first stage of labor. We particularly wanted to assess the influence of body mass index (BMI) on CS risk., Methods: A retrospective case-control study in a tertiary delivery unit with 5200 deliveries annually. Cases were 296 term primiparous women whose intended vaginal labor ended in acute CS because of dystocia. Controls were primiparas with successful vaginal delivery VD (n = 302). The data were retrieved from medical records. Multiple logistic regression analyses were used to assess the associations between BMI and covariates on labor dystocia., Results: In the cases ending with acute CS, women were older (OR 1.06 [1.03-1.10]), shorter (OR 0.94 [0.91-0.96]) and more often had a chronic disease (OR 1.60 [1.1-2.29]). In this group fetal malposition (OR 42.0 [19.2-91.9]) and chorioamnionitis (OR 10.9 [5.01-23.6]) were more common, labor was less often in an active phase (OR 3.37 [2.38-4.76]) and the cervix was not as well ripened (1.5 vs. 2.5 cm, OR 0.57 [0.48-0.67] on arrival at the birth unit. BMI was higher in the dystocia group (24.1 vs. 22.6 kg/m 2 , p < 0.001), and rising maternal pre-pregnancy BMI had a strong association with dystocia risk. If BMI increased by 1 kg/m 2 , the risk of CS was 10% elevated. Among obese primiparas, premature rupture of membranes, chorioamnionitis and induction of labor were more common. Their labors were less often in an active phase at hospital admission. Severely obese primiparas (BMI ≥ 35 kg/m 2 ) had 4 hours longer labor than normal-weight parturients., Conclusions: Labor dystocia is a multifactorial phenomenon in which the possibility to ameliorate the condition via medical treatment is limited. Hospital admission at an advanced stage of labor is recommended. Pre-pregnancy weight control in the population at reproductive age is essential, as a high BMI is strongly associated with labor dystocia.