More than 80% of the 3 million women who labor and deliver each year in the United States undergo continuous electronic fetal monitoring (EFM) during labor in order to fetal hypoxia and prevent the transition to acidemia, expedited operative delivery, and/or neonatal morbidity. Category II EFM is the most commonly observed group of fetal heart rate features in labor. One common response to Category II EFM is maternal oxygen (O2) supplementation. The theoretic rationale for O2 administration is that it increases O2 transfer to a hypoxic fetus. There are conflicting national guidelines regarding O2 administration - the American College of Obstetricians and Gynecologists suggest O2 is ineffective, whereas the Association of Women's Health, Obstetric, and Neonatal Nurses recommend continued use given lack of definitive data on safety and efficacy. A recent national survey of nearly 600 Labor & Delivery providers in February 2022 revealed that 49% still use O2 . Thus, there remains equipoise on the topic and high-quality data on the safety of intrapartum O2 is needed. None of the trials to date have studied the effect of intrapartum O2 on important clinical measures of neonatal or maternal morbidity. This safety data is imperative because the field of obstetrics must hold supplemental O2 to the same rigorous standards applied to any drug used in pregnancy. Without data on these definitive outcomes, it will be challenging to implement evidence-based recommendations for supplemental O2 use on Labor & Delivery. The investigators will conduct a large, multicenter, randomized noninferiority trial of O2 supplementation versus room air in patients with Category II EFM in labor.

Inclusion Criteria:

• Singleton gestation
• Gestational age>=37 weeks
• Spontaneous labor or induction of labor
• English or spanish speaking
• Planned continuous fetal monitoring

Exclusion Criteria:

• Preterm gestation
• Major fetal anomaly
• Multiple gestation
• Category III fetal monitoring at time of admission
• Maternal hypoxia <95%
• Planned or scheduled cesarean delivery Excluded from randomization if receiving nitrous oxide for analgesia at time of randomization.

The investigators will plan to share de-identified participant data and outcomes, study protocol, and analytic code via the NICHD Data and Specimen Hub (DASH).

Upon publication of primary analysis and planned secondary analyses.