Impact of body mass index on sonographic measurement of head perineum distance before operative vaginal delivery.

Objective: To evaluate the impact of body mass index (BMI) on sonographic measurement of head perineum distance (HPD) before operative vaginal delivery (OVD).

Methods: This was a single-center retrospective cohort study (Lille, France) conducted from March 1, 2019 to October 31, 2020 including all singleton and OVD. HPD measurement was systematically performed without and with compression on the perineum soft tissues. The level of station was defined by vaginal examination and three maternal BMI groups were defined (normal BMI [<24.9 kg/m] vs overweight [25-29.9 kg/m] vs obese [≥30 kg/m]). HPD measures were compared between BMI groups and compression, in distinct level of station, using a two-factor analysis of variance including BMI groups, the compression, and the interaction term BMI group compression.

Results: A total of 775 women were included: 488 with normal BMI, 181 overweight patients and 106 obese patients. The measurement of HPD before OVD without and with compression on the soft tissues was significantly different between the BMI groups only in the lower part, particularly between normal BMI and obese patients (mean difference (95% CI): 6.6 mm (4.0 to 9.2) without compression; 3.8 (1.1 to 6.4) with compression).

Conclusion: The values of HPD without and with compression on the soft tissues on the maternal perineum were different according to the maternal BMI concerning lower part station. Thus, it seems important to define thresholds of HPD measures corresponding to each head station levels according to maternal BMI.