This observational study characterizes the relationship between amniotic fluid lactate and uterine electromyography during labor in healthy individuals at term planning a vaginal birth. Additional comparison measures and outcomes measures will be collected and analyzed as exploratory measures.

Elevated amniotic fluid lactate (AFL) and uterine electromyography (EMG) mathematically transformed to calculated the power density spectrum median frequency have been found separately to be associated with labor dystocia and cesarean birth. Similarly, recent research indicates that the cytokine IL-6 may play a critical role in labor onset and potentially labor progression. All three are hypothesized to reflect that uterine fatigue plays a role in the pathophysiology of labor dystocia for some women.

This study aims to characterize the relationship between these three measures to better understand the role of uterine fatigue in labor dystocia and triangulate biomarkers of fatigue that may lead to a better understanding of labor dystocia phenotypes.

In addition to the primary measures, comparison measures will be collected. For AFL comparison, venous whole blood lactate and capillary lactate will be collected and measured using both a point-of-care lactate meter (Stat Strip Lactate Meter, Nova Biomedical) and a table-top blood gas analyzer (Nova Prime). For EMG comparison, routine contraction monitoring data will be collected and compared to the EMG signal. IL-6 will be measured both in blood and amniotic fluid using a novel microsampling protocol. Three visual analog scales on women's experiences of fatigue, anxiety, and pain will also be administered.

Additional data will be collected from the medical record to perform exploratory and hypothesis generating analysis.

Inclusion Criteria:

• Maternal age: 18-41 years at the due date
• Term gestation: 37-41 weeks estimated gestational age (EGA)
• Single fetus in a vertex presentation: Pregnancy with only one fetus that is in a head down position with the occiput leading into the pelvis upon admission to the hospital.

Exclusion Criteria:

• Pre-existing or gestational diabetes requiring medical management: The diagnosis of diabetes of any kind requiring management with medications including but not limited to insulin, glyburide, and metformin as indicated in the health record.
• Pre-existing or gestational hypertension or pre-eclampsia requiring medical management: The diagnosis of hypertension of any kind requiring management with medications including but not limited to long-term hypertension management such as methyldopa and short-term or emergency hypertension management such as labetalol or magnesium sulfate as indicated in the health record.
• Suspected fetal anomaly or intrauterine growth restriction: Any diagnosed fetal anomalies or intrauterine growth restriction as indicated in the health record.
• Use of amnioinfusion prior to sample collection: Amnioinfusion, the infusion of fluids into the uterus using an intrauterine catheter may affect AFL concentrations. Any amnioinfusion prior to sample collection will exclude participants from continued participation.
• Fetal distress prior to or during sample collection: Category 3 fetal heart tracing as defined by the National Institute of Child Health and Development22, absent fetal heart rate variability with recurrent late deceleration, recurrent variable decelerations, bradycardia, or sinusoidal rhythm.

Healthy pregnant women with a Singleton Term fetus in the Vertex position in labor.

Data sharing can be arranged by contacting the PI