Contributing factors in forceps associated pelvic floor trauma.

Levator avulsion occurs in 10-35% of women after a first vaginal delivery, with forceps being the main risk factor. Three mechanisms have been proposed to account for the high risk of trauma: the additional space requirement, increased speed of distension, and/ or greater force. This study examines the additional space requirements associated with forceps to determine any associated increase in avulsion risk. This was an in vitro simulation study of spatial requirements for delivery of a fetal head by forceps, with mathematical modelling of spatial requirements using data from a local observational study. A balloon device was used to model head circumferences through the range of expected measurements at term, with measurements taken after application of three different types of forceps. Each measurement was performed in triplicate. On average, forceps increased the circumference of the fetal head by 1.01 cm for Wrigley's, 1.04 cm for Kielland's, and 1.64 cm for Neville-Barnes forceps, resulting in an estimated increase in the diameter of the fetal head by 0.32 cm, 0.33 cm, and 0.52 cm, respectively. This increase was linear throughout the tested range. In 534 singleton pregnancies at term, we determined an odds ratio (OR) of 1.11 per centimeter head circumference for avulsion. Hence, the additional space requirement due to forceps explains ORs of 1.11, 1.12, and 1.19 for avulsion, depending on forceps type. The effect of forceps on avulsion risk is not fully explained by the increase in space requirement alone. Other factors, such as shortened time to maximum distension and/or increased peak force applied may contribute to the excess risk.