Asynclitism in the second stage of labor: prevalence, associations, and outcome.

Background: Fetal head asynclitism may affect labor progress, increase the need for obstetrical intervention, and even be associated with difficult or failed instrumental delivery. However, there is limited evidence on the true prevalence and associations of asynclitism when diagnosed by transperineal ultrasound in the second stage of labor.
Objective: This study aimed to examine the prevalence and outcome of asynclitism in the second stage of labor in a regional hospital in Hong Kong.
Study Design: This is a prospective cohort study involving 92 term nulliparous women with singleton pregnancy in the second stage of labor, recruited from December 2019 to December 2020. Transperineal ultrasound was performed and asynclitism was diagnosed if there was asymmetry of intracranial structures on the transverse plane. To assess the fetal head station, the head perineum distance was measured at rest and on pushing, and the mode of delivery was recorded. Relationship between asynclitism and fetal head position, and between head perineum distance and the mode of delivery, were evaluated with chi-squared tests and Mann-Whitney U tests.
Results: The prevalence of asynclitism was 15% (14 of 92), of which 12 were anterior asynclitism and 2 were posterior asynclitism. The prevalence of asynclitism was less common in occiput anterior compared with nonocciput anterior position (6.7% vs 53%, P<.01). Women with asynclitism were associated with smaller delta head perineum distance (head perineum distance at rest minus that at pushing) than women without asynclitism (median [interquartile range], 0.68 cm [0.85 cm] vs 0.91 cm [0.71 cm]; P=.01). Eventually, there was a trend of more operative deliveries in women with asynclitism (43%, or 6 of 14 women) than in women without asynclitism (27%, or 21 of 78 women), although this difference was not statistically significant (P=.22).
Conclusion: The prevalence of asynclitism at transperineal ultrasound was rather common in nulliparous women at second stage of labor and seemed more commonly associated with nonocciput anterior position.
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