Background
Determining fetal head descent, expressed as fetal head station and engagement is an essential part of monitoring progression in labor. Assessing fetal head station is based on the distal part of the fetal skull, whereas assessing engagement is based on the proximal part. Prerequisites for assisted vaginal birth are that the fetal head should be engaged and its lowermost part at or below the level of the ischial spines. The part of the fetal head above the pelvic inlet reflects the true descent of the largest diameter of the skull. In molded (reshaped) fetal heads, the leading bony part of the skull may be below the ischial spines while the largest diameter of the fetal skull still remains above the pelvic inlet. An attempt at assisted vaginal birth in such a situation would be associated with risks. Therefore, the vaginal or transperineal assessments of station should be supplemented with a transabdominal examination. We suggest a method for the assessment of fetal head descent with transabdominal ultrasound.
Objective
To investigate the correlation between transabdominal and transperineal assessment of fetal head descent, and to study fetal head shape at different labor stages and head positions.
Study Design
Women with term singleton cephalic pregnancies admitted to the labor ward for induction of labor or in spontaneous labor, at the Cairo University Hospital and Oslo University Hospital from December 2019 to December 2020 were included. Fetal head descent was assessed with transabdominal ultrasound as the suprapubic descent angle between a longitudinal line through the symphysis pubis and a line from the upper part of the symphysis pubis extending tangentially to the fetal skull. We compared measurements with transperineally assessed angle of progression and investigated interobserver agreement. We also measured the part of fetal head above and below the symphysis pubis at different labor stages.
Results
The study population comprised 123 women, of whom 19 (15%) were examined before induction of labor, 8 (7%) in the latent phase, 52 (42%) in the active first stage and 44 (36%) in the second stage. The suprapubic descent angle and the angle of progression could be measured in all cases. The correlation between the transabdominal and transperineal measurements was −0.90 (95% confidence interval, −0.86 to −0.93). Interobserver agreement was examined in 30 women and the intraclass correlation coefficient was 0.98 (95% confidence interval, 0.95–0.99). The limits of agreement were from −9.5 to 7.8 degrees. The fetal head was more elongated in occiput posterior position than in non-occiput posterior positions in the second stage of labor.
Conclusion
We present a novel method of examining fetal head descent by assessing the proximal part of the fetal skull with transabdominal ultrasound. The correlation with transperineal ultrasound measurements was strong, especially early in labor. The fetal head was elongated in the occiput posterior position during the second stage of labor.
Introduction
Determining fetal head descent, expressed as fetal station and engagement is an essential part of monitoring progression in labor. Assessing fetal head station is based on the distal part of the fetal skull, whereas assessing engagement is based on the proximal part. A prerequisite for assisted vaginal birth is that the fetal head station should be at the level of the ischial spines or lower. Another prerequisite is fetal head engagement, which occurs when the widest part of the fetal head has descended below the pelvic inlet and two fifths of the head or less is palpable above the brim. , Fetal head station correlates with fetal head engagement, but station does not always truly indicate fetal head engagement. In flexed occiput anterior (OA) position, fetal head engagement occurrs when the leading bony part of the skull is at the level of the ischial spines. In malpositions and molded (reshaped) fetal heads, the leading bony part of the skull may be below the ischial spines when the largest diameter of the fetal skull is still above the pelvic inlet, , An attempt at assisted vaginal birth in such a situation would be associated with risks of maternal and neonatal complications, and therefore, contraindicated in modern obstetrics. To avoid this mismanagement, a vaginal or transperineal assessment of fetal head descent should be supplemented with abdominal examination, as recommended in several guidelines and the World Health Organization partograph. , , Unfortunately, abdominal examination to determine the fifths of the head above the symphysis pubis was inexact and poorly reproducible. An objective abdominal assessment of the proximal fetal head descent is warranted.
Why was this study conducted?
Engagement is a prerequisite for operative vaginal attempts, and it occurs when the largest diameter of the fetal head passes the pelvic inlet. Fetal head molding (reshaping) may lead to overestimating descent by clinical vaginal and transperineal ultrasound examinations. Fetal head descent assessments should be supplemented with an abdominal examination. Because clinical abdominal examination has shown to be inaccurate, an objective transabdominal ultrasound assessment is warranted.
Key findings
Transabdominal ultrasound examination of fetal head descent was feasible in this study and the correlation between transabdominal and transperineal ultrasound measurements of fetal descent was strong ( r =−0.90; 95% confidence interval, −0.86 to −0.93). The fetal head was elongated in the second stage of labor in occiput posterior positions.
What does this add to what is known?
We present a novel method for examining fetal head descent by assessing the proximal part of the fetal skull with transabdominal ultrasound.
Distal fetal head descent can be examined with transperineal ultrasound and angle of progression (AoP) is a well-established ultrasound method. , , Transabdominal ultrasound examination of fetal head descent might be easier to perform for most clinicians and is not affected by fetal head molding. Moreover, women prefer transperineal ultrasound over clinical vaginal examinations, and transabdominal scanning might be even more acceptable to laboring women. A previous study investigated fetal head engagement with transabdominal ultrasound but failed to visualize the sacral promontory with ultrasound.
We suggest measuring the suprapubic descent angle (SDA) transabdominally, as the angle between a longitudinal midline through the symphysis pubis and a line from the upper part of the symphysis pubis extending tangentially to the fetal skull. We aimed to investigate the correlation between transabdominal and transperineal assessment of fetal head descent, and to study fetal head shape at different labor stages and head positions.
Materials and Methods
We conducted a prospective observational study in non-consecutive case series at Cairo University Hospital, Cairo, Egypt, and Oslo University Hospital, Oslo, Norway, from December 2019 to December 2020. Women were included when a member from the research team was on call. Women with uncomplicated singleton, cephalic, term pregnancies were eligible for recruitment. Women were recruited at the start of induction of labor, in the latent phase, or in the active stages of labor. A total of 83 women were included from Cairo and 40 from Oslo. All women were informed and consented to participate in the study. Ethical approval given by the Regional Committees for Medical and Health Research Ethics, Norway on February 12, 2018 (reference number 2018/176 /REK nord) and by the Research Scientific and Ethical Committee, Department of Obstetrics and Gynecology, Cairo University, Giza, Egypt, on October 20, 2019 (reference number O19005).
The ultrasound devices used for scanning were GE Voluson S6 or E10 (GE Medical Systems, Zipf, Austria) and Samsung Sonoace R3 (Samsung Medison, Seoul, Republic of Korea). We used transabdominal curvilinear transducers. First, a transabdominal scan was done to visualize the midsagittal plane. Then, the probe was placed vertically above the upper part of the symphysis pubis to enable visualization of the pubic bone. A longitudinal line was drawn through the symphysis pubis in the midline and a second line was drawn from the upper point of the symphysis pubis tangentially to the upper part of the fetal skull ( Figure 1 , Video 1 ). The upper and lower poles, the longitudinal contours and the central calcifications were used as references when the longitudinal line through the midline of the symphysis pubis was drawn. The symphysis pubis was orientated to look horizontal or slightly oblique on the ultrasound images. In fetuses with the head below the symphysis pubis the second line was drawn to the junction of the cervical spine and occipital bone ( Figure 2 , Video 2 ). The angle between the lines were measured as the SDA. We drew a line perpendicular to the upper point of the symphysis pubis (suprapubic line) and measured the distances from this line to the highest point of the fetal skull ( Figure 3 ), as suggested previously. The bladder should be empty.
Thereafter, the woman was placed in a semirecumbent position and a transperineal scan was performed. AoP was measured as the angle between a longitudinal line through the symphysis pubis and a line from the lowest point of the symphysis pubis tangentially to the fetal skull as previously described ( Figure 1 , Videos 3 and 4 ). The longitudinal line through the symphysis pubis was drawn as described above. We also measured the distance between a line perpendicular to the lowest point of the symphysis pubis (infrapubic line) and the lowest part of the fetal skull as previously described as progression distance ( Figure 3 ). The shape of the fetal head was assessed as the sum of the distance from the suprapubic line to the upper part of the fetal skull (D1) and the distance below the infrapubic line (D2), D1+D2 ( Figure 3 ).
Fetal head position was examined both transabdominally and transperineally by the same examiner sequentially. The occiput position was recorded in relation to a clock-face and categorized as OA (≥10 and ≤2 o’clock), left occiput transverse (LOT; >2 and <4 o’clock), occiput posterior (OP; ≥4 and ≤8 o’clock) and right occiput transverse positions (ROT; >8 and <10 o’clock). , The fetal spine, orbits, midline structures, and choroid plexus were used to determine the fetal position with ultrasound.
All measurements were done offline at a later stage. The interobserver variation was investigated by examiner 1 in Egypt (R.K.) and examiner 2 in Norway (T.M.E.). Examiner 1 measured short time after recording and examiner 2 measured at a later stage. None of them were informed about clinical outcomes. They examined 30 transabdominal scan images of the SDA, independently and blinded to each other’s measurements.
Statistical analysis
Categorical variables were compared with Fisher exact test and continuous variables were compared with t test and 1-way analysis of variance. The association between continuous variables was analyzed using linear regression and Pearson correlation coefficient. Interobserver agreement was expressed with the intraclass correlation coefficient and with limits of agreement and illustrated with a Bland-Altman plot. P values of <.05 were considered significant. Data were analyzed with the statistical software package SPSS Statistics, version 25.0 (IBM SPSS; IBM Corporation, Armonk, NY,) and with VassarStats ( http://vassarstats.net ).
Results
Study population
The study population comprised 123 women, of whom 19 (15%) were examined before induction of labor, 8 (7%) in the latent phase, 52 (42%) in the active first stage and 44 (36%) in the second stage. The mean maternal age was 29 (standard deviation [SD], 6) years, 70 (57%) were nulliparous women, mean body mass index was 28 (SD, 5) kg/m 2 and median gestational age was 278 (range, 259–296) days. Characteristics of fetal position and station differentiated according to labor phases and stages are shown in the Table .
| Characteristics | Before active phase (N=27) | First active stage (N=52) | Second active stage (N=44) |
|---|---|---|---|
| SDA (°) | 150 (11) | 134 (15) | 87 (20) |
| AoP (°) | 79 (9) | 98 (17) | 148 (18) |
| Correlation between SDA and AoP, r (95% CI) | −0.58 (−0.26 to −0.79) | −0.77 (−0.63 to −0.86) | −0.52 (−0.26 to −0.70) |
| Distance above suprapubic line (mm) | 49 (14) | 43 (15) | −4 (17) |
| Distance below the infrapubic line (mm) | −18 (14) | 5 (19) | 51 (14) |
| Sum of distance above the suprapubic line and distance below the infrapubic line (mm) | 31 (18) | 48 (21) | 47 (18) |
| OA position, n (%) | 5 (18.5) | 23 (44.2) | 36 (81.8) |
| OT position, n (%) | 14 (51.9) | 24 (46.2) | 1 (2.2) |
| OP position, n (%) | 8 (29.6) | 5 (9.6) | 7 (15.9) |
The SDA and the AoP could be measured in all cases. Figure 1 shows SDA and AoP with the fetal head at a high station before start of active labor (SDA=145° and AoP=90°) ( Videos 1 and 3 , respectively). Figure 2 shows a fetus at a low station where the uppermost part of the fetal head was at the level of the superior edge of the symphysis pubis (SDA=90° and AoP=157°) ( Videos 2 and 4 , respectively). Figure 4 shows a fetus moving under the symphysis pubis short time before the delivery ( Videos 5 and 6 ). The fetus is in OA position with an extended attitude (SDA=65° and AoP=150°).
