In this prospective observational double-blind study, the investigators aim to assess the effect of the single nucleotide polymorphism of the μ-opioid receptor gene (OPRM1, p.118A/G) on the duration of epidural fentanyl labor analgesia.

The single nucleotide polymorphism (SNP), p.118A/G of the µ -opioid receptor gene (OPRM1), may alter pain perception and individual response to opioid analgesia. This SNP leads to a variant µ-opioid receptor in which an asparagine is substituted for aspartate as the 102nd amino acid of the receptor protein (p.Asn102Asp). At the investigators' institution, it is common practice to initiate labor analgesia with epidural fentanyl. No data are available to determine how the µ-opioid polymorphism affects pain perception and response to opioids, such as fentanyl, in the investigators' population. The investigators aim to assess the effect of the single nucleotide polymorphism of the µ-opioid receptor gene (OPRM1, p.118A/G) on duration of epidural fentanyl labor analgesia.

This is a prospective observational and double-blind study. Before initiation of epidural labor analgesia, venous blood will be drawn into coded EDTA tubes. DNA isolation and genotyping of OPRM1:p.118A/G will be performed at the American University of Beirut. In 250 parturients, labor epidural analgesia will be initiated with 100 µg of epidural fentanyl following a test dose of lidocaine and epinephrine. No drugs will be injected until second request of analgesia. Patients will be divided into two groups based on their genetic groups: Group A consists of wild-type homozygote (A118), and Group G includes heterozygote and homozygote carrying the G118 allele. Duration of fentanyl analgesia (primary outcome) will be compared. Data will be presented as a mean ± SD or numbers and percentages. Statistical analysis will be performed with the Student's t-test, Chi-square or Fisher's exact test as appropriate. P value <0.05 will be considered significant.

Inclusion Criteria:

• Nulliparous parturients ≥ 37 weeks gestation, with ASA I or II, with an uncomplicated course of singleton vertex pregnancy (in spontaneous labor or with spontaneous rupture of membrane), requesting epidural analgesia for pain relief

Exclusion Criteria:

• Advanced cervical dilation (> 5 cm) at the time of request for epidural analgesia
• Severe medical or obstetric complications (preeclampsia, uncontrolled pregestational diabetes, etc…)
• Multiple gestation
• Contraindication to epidural analgesia
• Local anesthetics or fentanyl allergy
• History of chronic opioid use or recent acute opioid use.

Nulliparous parturients ≥ 37 weeks gestation, with ASA I or II, with an uncomplicated course of singleton vertex pregnancy requesting epidural analgesia for pain relief