BACKGROUND:

Perineal trauma after vaginal delivery can have significant long-term consequences. It is unknown if a larger infant head circumference or smaller maternal perineal anatomy are risk factors for perineal trauma after vaginal delivery.

METHODS:

We conducted a prospective cohort study of low-risk nulliparous women. Data collected included maternal characteristics, antepartum Pelvic Organ Prolapse Quantification measurements of the perineal body and genital hiatus, labor characteristics, perineal trauma, and infant head circumference. Perineal trauma was defined as trauma that extended into the muscles of the perineum (second-degree or deeper). Univariate and multivariate logistic models were created to calculate odds ratios (OR) and 95 percent confidence intervals (CI).

RESULTS:

We observed 448 vaginal births. Multivariate analysis demonstrated a significant association between infant head circumference at birth and perineal trauma: OR 1.22 for each increase of 1 cm in head circumference (95% CI 1.05-1.43). There was no association between perineal body or genital hiatus length and perineal trauma.

CONCLUSIONS:

In nulliparous low-risk women a larger infant head circumference at birth increases the likelihood of perineal trauma, although the effect is modest. Antenatal perineal body and genital hiatus measurements do not predict perineal trauma. These results do not support alteration in mode of delivery or other obstetric practices.