Large foetal head circumference observed on third-trimester ultrasound, as a predictor risk factor for high degree perineal traumas during a vaginal delivery: a prospective cohort study

Abstract

Background: Obstetric anal sphincter injuries (OASIS) are a serious complication that women suffer during vaginal deliveries. Once it has occurred, although obstetricians always suture it at the immediate postpartum, faecal and urinary incontinences, flatulence and pelvic organ prolapse can appear. To prevent it, some protective perineal measures are performed routinely during childbirth. Several researches have confirmed some of the risk factors of perineal traumas, but the majority of them are measured after birth. Hence, foetal head circumference has been proposed as a possible predictive risk factor of these lesions. Objectives: Assess whether the presence of a large foetal head circumference (FHC) assessed prenatally by ultrasound is associated to an increased risk of suffering a high degree perineal trauma during childbirth and/or an operative delivery among nulliparous and singleton gestation pregnant women. The cut-off point from which women have more risk of suffering a severe perineal lesion will be also determined. Design: It will be an observational prospective cohort study performed in the Hospital Universitari de Girona Doctor Josep Trueta (HJT) between June 2016 and February 2021. Participants: Using a non-probabilistic consecutive sampling, 3,080 nulliparous pregnant women older than 18 years and with a singleton gestation of 37 gestation weeks presented cephalically will be recruited. Methods: Patients will be divided depending on the value of their foetal head circumference observed on the third-trimester ultrasound screening is equal or greater than the 95th percentile (36.4 cm), or lower than it. At the immediate postpartum, we will assess which ones have suffered a high degree laceration. Main outcomes: Assess if a large foetal head circumference (FHC) assessed prenatally by ultrasound is a predictive factor of suffering a severe perineal tear during a vaginal delivery and if it increases the rates of the assisted births. Moreover, find the cut-off point from which there is more risk to suffer this disease. Statistical analysis: we will use a Chi-square test to compare the exposed and the non-exposed groups and study whether a large foetal head circumference is a predictive risk of suffering a high degree perineal trauma and also a factor that increases the rates of operative deliveries. To find a cut-off point of FHC from which there is an incremented risk for having a severe perineal tear, we will use a Younden index based on a ROC analysis. Regarding the multivariate analysis, we will adjust the independent variables for covariables by using a multiple logistic regression model. Finally, sensitivity analysis will be performed to assess how the unplanned caesareans due to failure to progress can interfere in our results