Quantification of fetal head direction and descent

Authors


(e-mail: tutschek@me.com)

We read with interest the letter by Dr Iliescu et al.[1] about a simplification of the assessment of head direction as depicted by intrapartum translabial ultrasound (ITU). We would like to caution against introducing supposedly easier approaches (angles) for intrapartum head descent.

As well as head station and head descent, we proposed the ‘head direction’ as a simple measurement[2, 3] (Figure 1). This was based on data from a pilot study obtained during the first and second stages of normal labor as well as immediately prior to operative vaginal delivery. We pointed out that, often, the internal structures of the fetal head could not be seen clearly from a translabial insonation. We therefore proposed measurement of the angle between the longest visible axis of the fetal head and the long axis of the pubic symphysis as ‘head direction’[2]. We also proposed the three head direction classes: ‘head down’ (angle < 0°), ‘horizontal’ (angle 0–30°) and ‘head up’ (angle > 30°) and correlated them with delivery.

Figure 1.

Parameters measured on intrapartum translabial ultrasound (ITU). Mid-sagittal translabial ultrasound shows the fetal head and the long axis and lower margin of the pubic symphysis. The infrapubic line is perpendicular to the long axis of the pubic symphysis. A parallel plane through the ischial spines is 3 cm below the infrapubic line; it is used to measure head station (‘ITU station’). ‘Head direction’ (degrees upwards or downwards with regard to the symphysis) and the ‘angle of descent’ (tangent to the fetal head from the lower margin of the pubic symphysis) are the other two quantitative parameters. The asterisks indicate the contour of the caput succedaneum.

In a recent study[3], we showed that head direction and the dynamics of head descent during a contraction and over the course of delivery depend on the absolute head station: at higher stations, head direction tends to be ‘horizontal’ or even ‘down’, but at ITU-measured head stations of 0 to +2, a marked upward change in direction occurs during a contraction (see videos S1 and S2 in Tutschek et al.[3]). Head station is measured as shown in Figure 1. In the letter by Dr Iliescu et al., Figure 1a shows a high head station: the internal structures of the fetal head do not permit identification of the midline structures that indicate the correct biparietal diameter (BPD) plane. In fact, a correct BPD section cannot usually be obtained from this insonation; it is only at lower stations, such as depicted in their Figure 1b, that the midline structures become apparent.

In the letter by Dr Iliescu et al., Figure 2a does not show the head direction measurement as we initially proposed it. In their Figure 2b, two lines, one crossing the pubic bone in an oblique fashion, one through the BPD, form yet another angle.

For clarification, the measurement of ITU head station requires two steps (Figure 1): (1) measuring the angle defined by two easily obtainable axes (pubic axis and head direction); (2) measuring the distance from the intersection of the head direction and the infrapubic line to the deepest bony part of the skull[3]. ITU head station is an objective measurement, expressed on the same scale with regard to the ischial spines as is the classical digital palpation.

We also showed that the angle of descent correlates linearly with head station, allowing a simple conversion between the two[3] (Table 1). The value of this conversion was recently appreciated in another article in UOG[4]. The angle of descent is also easy to obtain and is reproducible[3, 5].

Table 1. Linear conversion between intrapartum translabial ultrasound (ITU) parameters ‘angle of descent’ (AoD) and ‘head station’
AoD ()Head station (cm)AoD ()Head station (cm)
  1. ITU head station (in cm) = (AoD (in degrees) × 0.0937) – 10.911. (Data from Tutschek et al.[3])
84−3.01321.5
90−2.51382.0
95−2.01432.5
100−1.51483.0
106−1.01543.5
111−0.51594.0
1160.01644.5
1220.51705.0
1271.0

We suggest using the parameter ‘head direction’ – in addition to head station – as we initially described it, because it enables a simple assessment ‘at a glance’. We currently assess ITU head direction clinically before almost all operative vaginal deliveries.

On this occasion, we would also like to stress the importance of the dynamic changes that occur during contraction. The changes in ITU parameters at the height of a contraction, augmented by maternal pushing, add important information in comparison to the assessment of only a static image at rest. We always assess ITU parameters with the patient at rest and at the height of a contraction.

  • B. Tutschek†‡, T. Braun§, F. Chantraine and

  • W. Henrich§

  • Center for Fetal Medicine and Gynecological

  • Ultrasound, Freie Strasse 38, 4001, Basel, Switzerland;

  • Medical Faculty, Heinrich Heine University,

  • Düsseldorf, Germany; §Charité, Campus Virchow,

  • University Hospital, Berlin, Germany; Obstetrics and

  • Gynecology, University Hospital, Liège, Belgium

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