This study seeks to determine the effectiveness of nitrous oxide in reducing the anxiety and pain associated with external cephalic version (ECV); a maneuver to turn the fetus from breech to cephalic position.

Study Procedures:

Potential subjects will be identified as they are scheduled to undergo ECV at the UNC Department of Obstetrics and Gynecology. Subjects will be enrolled in the study by one of the study investigators. Details of the study will be reviewed with potential subjects and those wishing to enroll in the study will be consented for the study by one of the study investigators. Once enrolled participants will be randomized to receive either a mixture of 50% nitrous oxide in 50% oxygen or 100% oxygen, both of which will be delivered via the Nitronox apparatus. Demographic variables will be recorded for each participant at the time they are enrolled in the study

The Nitronox apparatus will be set up to deliver either 50% nitrous oxide in 50% oxygen or 100% oxygen by an anesthesia provider covering labor and delivery who is not involved in the study. The Nitronox device will be covered to prevent participants, study investigators and obstetricians performing the procedure from knowing whether nitrous oxide is being delivered. Nitrous oxide is odorless and colorless, so in that way is indistinguishable from oxygen.

Participants will be prepared for ECV according to current protocols for ECV at UNC. IV access will be obtained and tocolytic agents will be given according to current practice and protocols. Monitoring of maternal and fetal vital signs will be done according to current protocols.

Immediately before an ECV attempt participants will breathe through the facemask of the Nitronox device for 30 seconds and will then be free to use the device ad lib during the ECV attempt. Immediately following each attempt, while fetal monitoring is occurring, the participants will be asked to rate their maximum pain score during the previous attempt. They will also be asked to rate their current level of anxiety. This process will be repeated for any further ECV attempts.

At conclusion of all ECV attempts, while fetal monitoring is occurring, the participants will be asked to rate their current level of pain, their satisfaction with the procedure and about any side effects they experienced (nausea, vomiting, dizziness, headache, other). Participants will continue to be monitored on labor and delivery as current protocol dictates, typically 30 minutes of maternal and fetal monitoring if there are no complications and fetal status is reassuring.

Following the procedure, the obstetrician who performed the procedure will be asked to rate how difficult they felt it was to perform the procedure and to rate how much they felt the analgesic provided assisted in performing the procedure. They will also be asked to indicate whether they thought the patient was in the treatment or placebo arm of the study. This will be recorded to address the adequacy of blinding.

All study variables will be assessed and recorded by one of the study investigators to ensure consistency in data collection.

Inclusion Criteria:

• Female 18 years or greater
• Scheduled to undergo external cephalic version due to singleton breech presentation
• Not scheduled to have spinal or epidural anesthesia during the version procedure
• American Society of Anesthesiology (ASA) Physical Status 1, 2 or 3.
• Able to provide informed consent and adhere to study protocol

Exclusion Criteria:

• Current or historical evidence of any clinically significant disease or condition, especially cardiovascular or neurological conditions that, in the opinion of the Investigator, may increase the risk of exposure to nitrous oxide or complicate the subject's post-procedural course.
• Significant medical conditions or laboratory results that, in the opinion of the Investigator, indicate an increased vulnerability to study drugs and procedures.

Only females will participate because only females can be pregnant.

De-identified IPD for all primary and secondary outcome measures will be made available.

9-36 months after publication

Approval from an IRB, IEC, or REB and an executed data use/sharing agreement with UNC.

During each version procedure (or attempt), a total average of up to approximately 30 minutes and through approximately 15 minutes post-procedure following the final version attempt.

The Nitronox device will be covered. The person setting up the device with either 100% oxygen or 50% nitrous/50% oxygen, will not be participating in the study. The patient, investigator and labor and delivery personnel (obstetrician and nursing staff) will be blinded as to the medication being administered during the procedure.