Fetal heart rate (FHR) monitoring during labor aims to detect fetuses at risk of intrapartum hypoxia in order to accelerate their birth before the constitution of sequelae such as anoxic encephalopathy and cerebral palsy. But the positive predictive value of FHR monitoring is low, and many second-line tests have been proposed but none of them has been proven to be conclusive. Measure of placental elasticity and viscosity during labor could be a new second line test to complete the FHR monitoring. Several studies have shown that placental elasticity is increased in case of gestational diabetes, preeclampsia, or intrauterine growth restriction, but no study has focused on placental elasticity during labor. The investigators hypothesize that placental elasticity and viscosity could reflect the quality of maternal-fetal exchanges during labor, and could help to detect fetus at risk of intrapartum hypoxia.

The first aim of this study is to describe the values of placental elasticity and viscosity during different points in the labor: beginning of the labor, beginning of the active phase of the first stage of labor, and passive phase of the second stage of labor.

Secondary aims of this study are: to describe the values of placental elasticity and viscosity in case of oxytocin infusion, in case of abnormal FHR, and ex-vivo after placental expulsion.

This unicentric observational prospective study will include 150 patients with singleton pregnancy, without pathology, with spontaneous labor at term.

Inclusion Criteria:

• singleton pregnancy
• spontaneous labor at term
• body mass index < 30 at the beginning of the pregnancy
• anterior, lateral or fundal placenta
• epidural analgesia at the beginning of the labor
• normal fetal heart rate at the beginning of the labor

Exclusion Criteria:

• gestational diabetes
• preeclampsia
• intrauterine growth restriction