Evaluating fetal head dimension changes during labor using open magnetic resonance imaging

Fetal skull molding is important for the adaptation of the head to the birth canal during vaginal delivery. Importantly, the fetal head must rotate around the maternal symphysis pubis. The goals of this analysis were to observe a human birth in real-time using an open magnetic resonance imaging (MRI) scanner and describe the fetal head configuration during expulsion.

Real-time cinematic MRI series (TSE single-shot sequence, TR 1600 ms, TE 150 ms) were acquired from the midsagittal plane of the maternal pelvis during the active second stage of labor at 37 weeks of gestation. Frame-by-frame analyses were performed to measure the frontooccipital diameter (FOD) and distance from the vertex to the base of the fetal skull.

During vaginal delivery in an occiput anterior position, the initial FOD was 10.3 cm. When expulsion began, the fetal skull was deformed and elongated, with the FOD increasing to 10.8 cm and 11.2 cm at crowning. In contrast, the distance from the vertex to the base of the skull was reduced from 6.4 cm to 5.6 cm at expulsion.

Fetal head molding is the change in the fetal head due to the forces of labor. The biomechanics of this process are poorly understood. Our visualization of the normal mechanism of late second-stage labor shows that MRI technology can for the first time help define the changes in the diameters of the fetal head during active labor.