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Background & Objective: Labor pain is one of the most severe pains that mothers experience. Intense pain leads to stress and has unfavorable effects on the mother and fetus. This research was done to determine the effect of using Entonox (N2O) self administration on reducing labor pain in the active phase of labor& delivery, Mode of delivery, and newborn's Apgar score.
Methods: One hundred and twenty primiparous women in 29 Bahman Hospital in Tabriz were included it this randomly single-blind research. In the active phase of delivery, Entonox and Oxygen gases were used. The severity of the mother's pain was asked and was scored according to analogue criterion.
Results: There was a significant difference in frequency of pain intensity in two groups of Entonox and Oxygen. The Mean score of pain intensity in Entonex user, in the first & second stage of labor was 5.93% and 5.82% respectively, while for the oxygen users it was 6.99% and 6.74% which was significant. 41.7% of the mothers in the Entonox group had a severe pain and 11.7% had a very severe pain, whereas in the Oxygen group 58.3% of the mothers had a severe pain and 25% of them had very severe pain (p< 0.001). Meanwhile we found no significant difference between modes of delivery progress of labor and newborn's Apgar
score.
Conclusion: According to research results, Entonox reduces the intensity of labor pain more than oxygen and is effective during the first and second phase of delivery.

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Background & objectives: In many medical studies, the response variable is measured repeatedly over time to evaluate the treatment effect that is known as longitudinal study. The analysis method for this type of data is repeated measures ANOVA that uses only one correlation structure and the results are not valid with inappropriate correlation structure. To avoid this problem, a convenient alternative is mixed models. So, the aim of this study was to compare of mixed and repeated measurement models for examination of the Entonox effect on the labor pain.

Methods: This experimental study was designed to compare the effect of Entonox and oxygen inhalation on pain relief between two groups. Data were analyzed using repeated measurement and mixed models with different correlation structures. Selection and comparison of proper correlation structures performed using Akaike information criterion, Bayesian information criterion and restricted log-likelihood. Data were analyzed using SPSS-22.

Results: Results of our study showed that all variables containing analgesia methods, labor duration of the first and second stages, and time were significant in these tests. In mixed model, heterogeneous first-order autoregressive, first-order autoregressive, heterogeneous Toeplitz and unstructured correlation structures were recognized as the best structures. Also, all variables were significant in these structures. Unstructured variance covariance matrix was recognized as the worst structure and labor duration of the first and second stages was not significant in this structure.

Conclusions: This study showed that the Entonox inhalation has a significant effect on pain relief in primiparous and it is confirmed by all of the models.