We appreciate the interest in our paper and would like to take this opportunity to thank the authors for their effort to transmit a clear message to the general reader.
The authors remind us of the effective protocol with which to quantify ‘at a glance’ fetal head direction and the calculation of head station using intrapartum translabial ultrasound (ITU), and caution against introducing an angular evaluation of head direction. However, by definition, head direction is an angular measurement and recent studies[1, 2] and reviews[3, 4] have also described and exemplified head direction or direction angle as an angular measurement in two-/three-dimensional assessment.
It was not our intention in our Letter to propose another protocol for the use of a head direction parameter during labor. Rather we describe a simplification of the measurement technique, because we believe that in certain situations exact quantification may be necessary, for example, in differentiating borderline situations, such as in head-up/horizontal fetal positions (approximately 30°) or head-down/horizontal positions (around 0°). It may also prove useful for teaching or research purposes.
Regarding the visualization of the midline structures of the fetal head that permit identification of the biparietal diameter plane, we would like to emphasize the importance of the insonation angle in the midsagittal infrapubic plane. The midline structures are easily apparent at lower stations, when the transducer is orientated in a standard fashion, parallel to the pubic symphysis, as described by the authors and in Figure 1b in our Letter, because the structures are not shadowed by the pubic bone. The midline is also visualized at higher head stations if the transducer is orientated obliquely and caudal to the symphysis long axis in the same plane, as seen in Figure 2 of our Letter. We were not the first to use this insonation. ITU studies performed before active labor[7, 8] used this approach in order to avoid the interposition of the bony pubic structure in front of the fetal head landmarks. However, a different insonation, but in the same sagittal perineal infrapubic plane, cannot alter the results of ITU measurements, as the relation between the pubic bone and the fetal head remains the same. In fact, a search through the literature reveals variations between published images with respect to insonation in the infrapubic plane. The reason we used this ‘atypical’ insonation was to exemplify the technique in both low and high fetal head stations. Indeed, this insonation is not necessary in active labor and is not appropriate to evaluate ‘at a glance’ the head direction pattern, as the symphysis pubis long axis is not horizontal.
Regarding the suggestion by the authors that the image Figure 1a in our Letter is not appropriate to show head direction, in our opinion it is very similar to Figure 5 of the paper initially published by these authors, in which the ‘head-down’ direction is presented.
As suggested in this Correspondence, examiners should be made aware that the optimal way to use the head direction parameter clinically requires insonation parallel to the pubic symphysis long axis. If needed, a rapid one-step assessment of the direction angle can be made. As with other ITU parameters (progression angle, progression distance and head to perineum distance[10, 11]) that have been investigated and proven useful in women before labor, intrapartum head direction can be quantified, for research purposes, even at high stations using an oblique insonation.