Can Labor Pain be Reduced by Open Placebo Intervention? A Bayesian Account for Labor Pain

The human nervous system continuously processes a potentially overwhelming stream of signals from bodily senses, which induces uncertainty. To reduce this uncertainty, the brain transforms sensory input into a reliable perception of the world. According to the Bayesian hypothesis of perception, based on probabilistic estimation, conscious perception is shaped by both the current state of sensory information and by past experiences, associative learning, stored memories, and beliefs. This framework is particularly relevant for explaining the phenomenon of pain, including individual differences in pain perception and response. The focus of the present work is pain during childbirth, representing a major acute pain event. Pain during childbirth is a complex and unique experience with both sensory and emotional components, embedded within a defined physiological process. A key concept of the Bayesian model related to pain perception is the placebo phenomenon, which offers a compelling explanation for the analgesic placebo response. The overarching aim of this study is to evaluate whether the Bayesian model can account for changes in pain perception during labor following open placebo administration, as well as the involvement of expectations and personality traits. The study will be conducted at Hillel-Yaffe Medical Center, with the participation of 120 pregnant women. The research will be divided into two stages. The first stage will occur during the prenatal period, while participants are in a pain-free state. During this stage, questionnaires will be completed to assess expectations and personality traits, and the Focused Analgesia Selection Test (FAST) will be administered to evaluate within-subject variability in pain intensity. The second stage will take place during labor and will involve an open placebo intervention, during which pain levels and analgesic consumption will be monitored.

Aims and Hypotheses Overall Aim: To assess whether the Bayesian model can explain changes in pain perception during labor following open-placebo administration. Specific Aims and Hypotheses: Aim 1: To investigate whether open placebo reduces pain and analgesic consumption during labor. Hypothesis 1: As most evidence supporting the efficacy of open placebo originates from chronic pain contexts, and given the intensity of labor pain, this hypothesis is bidirectional-open placebo may or may not modulate pain and analgesic consumption during labor at the group level. Aim 2: To examine whether FAST outcomes predict changes in pain and analgesic consumption during labor following open-placebo administration. Hypothesis 2: FAST outcomes will predict changes in pain and analgesic use. Lower within-subject variability in pain reports during FAST will be associated with smaller changes in pain and analgesic consumption following open placebo. Aim 3: To explore the role of expectations and personality traits in placebo analgesia and their potential moderating effects on the relationship between FAST outcomes and the placebo response during labor. Hypothesis 3.1: Personality traits associated with the prior function will moderate the relationship between FAST outcomes and the placebo response. Hypothesis 3.2: Personality traits associated with the likelihood function will not moderate this relationship. Methods 1. Participants: Participants will include pregnant women admitted to Hillel Yaffe Medical Center. Based on power analysis (t-test, medium effect size = 0.5, α = 0.05, power = 0.8, allocation ratio = 0.666), a sample size of 106 is required. To compensate for potential dropouts, 120 participants will be recruited. Inclusion criteria: Singleton pregnancy after 24 weeks of gestation No contraindications for analgesia or vaginal birth Exclusion criteria: Obstetric emergencies such as intrauterine death, suspected placental separation, uterine rupture, placenta previa, or scheduled cesarean section 2. Tools and Procedures 2.1. Pain Assessment Pain will be measured using the Visual Analogue Scale (VAS), a unidimensional tool asking participants to rate pain from 0 ("no pain") to 10 ("worst imaginable pain"). Assessments will occur every 60 minutes throughout labor. 2.2. Analgesic Consumption Assessment Analgesia at Hillel Yaffe Medical Center includes intravenous Pethidine and epidural anesthesia. Nitrous oxide is not available due to infrastructure limitations. Analgesic usage will be tracked via medical records and epidural pump data, including usage duration, PCA button presses, and administered doses. 2.3. FAST Assessment The Focused Analgesia Selection Test (FAST) evaluates variability in response to noxious thermal stimuli of varying intensities, applied to the ventral surface of the non-dominant arm using a thermal sensory analyzer. Pain ratings (0-100 scale) are recorded after each stimulus delivered every 20 seconds. Variability is quantified using R² and ICC values. Lower variability indicates higher confidence in sensory input. Test duration is approximately 20 minutes. FAST outcomes are expected to positively correlate with changes in clinical pain. 2.4. Open Placebo Intervention Upon admission to the delivery room, participants receive standard Ringer solution. Participants will be randomly assigned to one of two groups: Open Placebo Group: The midwife will explain that although the infusion contains no medication, previous research has shown it may reduce pain. Control Group: The standard explanation will be provided, stating the fluids are intended only to maintain venous access. Placebo response will be assessed through: Differences in pain intensity (area under the curve of VAS scores over time) between groups Differences in analgesic consumption, particularly fentanyl dosage via PCA, between groups Analyses will also consider placebo response during distinct labor stages. 2.5. Expectations Assessment Expectations will be measured using two questionnaires: A short-form expectation questionnaire with items on treatment expectations and hopes. A childbirth-specific questionnaire measuring fear and expectations surrounding birth, using a 6-point Likert scale, with higher scores indicating greater fear. 2.6. Personality Characteristics Assessment Suggestibility: Measured using the Short Suggestibility Scale (SSS), consisting of 21 items. Higher scores indicate greater suggestibility. Anxiety: Assessed with the GAD-7, a 7-item scale ranging from 0 to 3 per item, with a total score from 0 to 21. Higher scores reflect more severe anxiety symptoms. Interoception: Evaluated with the Multidimensional Assessment of Interoceptive Awareness (MAIA), comprising 32 items across eight subscales. Items are rated on a 6-point Likert scale. Body Awareness: Measured using the Body Awareness Questionnaire, an 18-item scale using a 7-point Likert format. Higher scores indicate increased sensitivity to bodily cues. Self-Awareness: Evaluated using the short version of the Self-Consciousness Scale-Revised (SCS-R), with 23 items assessing private and public self-awareness on a 4-point scale. 3. Study Design The study will be conducted in the Obstetrics Division at Hillel Yaffe Medical Center. Following ethical and regulatory approval, participant recruitment will begin. Phase 1: Conducted prior to labor, when participants are pain-free. Participants will complete demographic, medical, and psychological questionnaires and undergo FAST to measure variability in pain reporting. Phase 2: Conducted during labor. Participants will be randomized into control or open placebo groups using a 2:1 allocation ratio (favoring the open placebo group). This ratio supports the analyses in Aims 2 and 3. Randomization will be double-blinded; allocation will be concealed using envelopes and administered by independent personnel not involved in labor care or pain assessments. Pain intensity will be recorded hourly using VAS. Participation concludes post-delivery, before transfer to the maternity ward.