Increased diagnostic accuracy of fetal head station by use of transabdominal ultrasound

Sir, A prerequisite for operative vaginal delivery is that the presenting part of the fetal head is at or below the level of the ischial spines. However, this only serves as a proxy for the true purpose of diagnosing fetal head station: to decide how much of the skull is left above the pelvic inlet. To perform an operative vaginal delivery safely, the biparietal (widest) diameter of the fetal skull must have passed the pelvic inlet, ie, be engaged in the pelvis. If the biparietal diameter is above the pelvic inlet, any operative vaginal delivery would be classified as high, which is contraindicated in modern obstetrics. In a flexed vertex presentation, the biparietal diameter is below the pelvic inlet when the presenting bony part of the fetal skull is at the level of the ischial spines. With malpresentations or severe moulding of the fetal skull, the presenting part may be below the ischial spines, but the biparietal diameter still above the pelvic inlet. To ensure accurate diagnosis of fetal head station, a vaginal examination should be supplemented with an abdominal examination to assess how much of the fetal head is palpable above the symphysis pubis.1 When the biparietal diameter has passed the pelvic inlet, no more than two-fifths of the fetal head is palpable above the symphysis pubis. Unfortunately this clinical method of determining level of fetal head is found to be inexact and poorly reproducible.2 Fetal station can be assessed with ultrasound and several transperineal techniques are published.3 Dietz et al referred fetal station to a line perpendicular to the lower part of the symphysis pubis.4 A transabdominal approach examining fetal station with ultrasound is desirable. Sherer et al assessed fetal head engagement with transabdominal ultrasound. The transducer was placed transversely above the maternal symphysis pubis and directed towards the maternal sacral promontory. The fetal head was considered being engaged when the biparietal diameter was located below a line extending between the superior aspect of the symphysis pubis and the sacral promontory.5 Unfortunately, the promontory is usually not visible due to shadowing from the fetal head. We suggest that fetal head engagement can be examined using sagittal transabdominal scanning and referring station to a line from the upper part of the symphysis perpendicular to a longitudinal line through the symphysis. We can then measure in millimeters how much of the fetal head is above the pelvic brim. The distance to the lowermost part of the skull can also be measured, and the proportion of the fetal head above the symphysis can be calculated objectively (Figure 1). Drop-outs occur when the ultrasound beams are parallel to the skull contour and the calipers should be placed at the outer skull where the contour is visible. The method is quick and easy to perform, and has the potential to increase safety in operative vaginal deliveries.