Correlation of intrapartum translabial ultrasound parameters with computed tomographic 3D reconstruction of the female pelvis

Descent of the presenting part assessed with ultrasound

Fetal head descent can be expressed as fetal station and engagement. Station is traditionally based on clinical vaginal examination of the distal part of the fetal skull and related to the level of the ischial spines. Engagement is based on a transabdominal examination of the proximal part of the fetal head above the pelvic inlet. Clinical examinations are subjective, and objective measurements of descent are warranted. Ultrasound is a feasible diagnostic tool in labor, and fetal lie, station, position, presentation, and attitude can be examined. This review presents an overview of fetal descent examined with ultrasound. Ultrasound was first introduced for examining fetal descent in 1977. The distance from the sacral tip to the fetal skull was measured with A-mode ultrasound, but more convenient transperineal methods have since been published. Of those, progression distance, angle of progression, and head-symphysis distance are examined in the sagittal plane, using the inferior part of the symphysis pubis as reference point. Head-perineum distance is measured in the frontal plane (transverse transperineal scan) as the shortest distance from perineum to the fetal skull, representing the remaining part of the birth canal for the fetus to pass. At high stations, the fetal head is directed downward, followed with a horizontal and then an upward direction when the fetus descends in the birth canal and deflexes the head. Head descent may be assessed transabdominally with ultrasound and measured as the suprapubic descent angle. Many observational studies have shown that fetal descent assessed with ultrasound can predict labor outcome before induction of labor, as an admission test, and during the first and second stage of labor. Labor progress can also be examined longitudinally. The International Society of Ultrasound in Obstetrics and Gynecology recommends using ultrasound in women with prolonged or arrested first or second stage of labor, when malpositions or malpresentations are suspected, and before an operative vaginal delivery. One single ultrasound parameter cannot tell for sure whether an instrumental delivery is going to be successful. Information about station and position is a prerequisite, but head direction, presentation, and attitude also should be considered.

The Value of Fetal Head Station as a Delivery Mode Predictor in Primiparous Women at Term before the Onset of Labor

Objective: Our objective was to demonstrate the role of the clinical determination of fetal head station (FHS) at term to predict the delivery mode in primiparous women before the onset of labor. Methods: This prospective study included unselected primiparous women at term who presented at our tertiary maternity. We excluded multiparous patients, pregnancies with a planned Cesarean section, non-cephalic presentations, and multiple pregnancies. The protocol included weekly clinical examinations to assess the FHS. The results were used to describe the clinical fetal head descent at term. We correlated the fetal head station determinations at each week with labor outcome, including the evaluations performed within the week before delivery. Results: The data show no significant differences between vaginal (VD) and Cesarean section delivery (CS) cases regarding FHS determined at each week at term. The median determinations at the gestational ages (GW) from 37 to 41 were −2 and −3, similar between the two groups, with a more consistent difference at 41 GW: station -1 for VD compared to −3 for CS. There were significant differences between the “week before delivery” evaluations of the two groups. The determinations showed for both groups similar minimum (−5), maximum (+1), and median (−2) FHS values. Most vaginal deliveries cases presented at weekly examinations with increasing rates toward more advanced stations: from 10% at station −4 to 35% at station −1. Although we investigated a low-risk group, we found significant differences between the vaginal and Cesarean groups in terms of age, weight, and BMI. We provided a multiple logistic regression equation that considered the predictive clinical variables at term: the fetal head situation, age, weight, height, and BMI. Conclusion: The clinical evaluation of fetal head station in primiparous before labor onset has a limited value regarding the prediction of the delivery mode. There is a potential benefit for the determinations performed within the week before delivery, but such a policy would require weekly assessments of the FHS at term, which is unlikely to be implemented. Another potential benefit would involve estimating labor outcomes in late-term or prolonged pregnancy. The fine tuning of the logistic prediction should be achieved by increasing the studied population and the number of centers involved before counseling primiparous women at term based on the clinical fetal engagement data.

Effects of Sex, Age and Height on Symphysis–Ischial Spine Distance Measured on a Pelvic CT

Objective: To examine the influence of age, sex and height on the symphysis–ischial spine distance (SID) measured on pelvic Computed tomography (CT)images in subjects of reproductive age, and to determine the interobserver reproducibility. This measurement (SID) is of great importance because the use of intrapartum ultrasound is based on the assumption of a specific value (30 mm) of such a measurement. Methods: This was a cross-sectional descriptive study in which SID was measured in subjects aged 20 to 44 years who had been scheduled for pelvic CT at our centre from January 2018 to May 2021 for different reasons. Radiographic measurements of the pelvis were obtained through the multiplanar reconstruction of the CT image. The images obtained from all of the participants were independently assessed by three senior radiologists, and the SID measurements made by each one were blinded from those of the remaining observers. Correlations between the SID and patient age, height and sex were analyzed by univariate and multivariate linear regression. Results: The mean SID for 87 of the enrolled participants (45 women, 42 men) was 28.2 ± 6.25 mm. Among the observers, the mean difference in this distance was 1 to 2 mm, and was scarcely related to measurement size, with agreement being greater than 70%. The mean SID was significantly related to sex and height (SID = −24.9 − 6.51 × sex (0 or 1) + 0.34 × height (cm); p = 0.01; sex equals 1 for a man and 0 for a woman), such that it was a mean of 2.5 mm greater in women than men (29.50 mm vs. 26.99 mm). Conclusion: Measurements of SID on CT images show good interobserver reproducibility, and are related to sex and height.

ISUOG Practice Guidelines: intrapartum ultrasound

The purpose of these Guidelines is to review the published techniques of ultrasound in labor and their practical applications, to summarize the level of evidence regarding the use of ultrasound in labor and to provide guidance to practitioners on when ultrasound in labor is clinically indicated and how the sonographic findings may affect labor management. We do not imply or suggest that ultrasound in labor is a necessary standard of care.