Digital vaginal examination is considered as the gold standard in evaluating fetal head progression, although it is subjective with many limitations [17,18,19]. TPUS is a promising tool in labor monitoring [20,21,22,23]. It gives objective data on the dynamics of labor and predicting the outcome of the operative vaginal delivery [18, 20].
In this study, TPUS was done in a short time with less discomfort to the pregnant ladies as reported by all of the participants compared with the digital vaginal examination. Only few previous studies support this finding [12, 17, 21].
Previous US studies evaluated the descent of the fetal head using the transabdominal and TPUS [18, 20, 22]. FHPD was evaluated in other studies for women with pre-labor rupture of membranes. It was found that a distance < 3 cm was associated with spontaneous vaginal delivery. This distance can also predict fetal head engagement if it was ≤ 6 cm with a sensitivity and specificity of 100% and 91% respectively [22]. They reported the inaccuracy of digital vaginal examination in monitoring progression of the fetal head during the first stage compared with the transperineal US, which is not affected by the presence of molding or caput succedaneum.
As a guide for digital vaginal exanimation, the space from the perineal surface to the ischial spine reaches 5 cm according to the guides of the WHO regarding stages of head descent. Torkildsen et al. [23] reported a cutoff value of 4.5 cm to define head engagement, which means the passage of the head of the fetus beneath the level of the ischial spine, the narrowest part of the birth canal. This agreed with the current study, whereas with a short FHPD distance of 3 cm ± 1.2 cm, the fate of labor was vaginal delivery; and with a distance of 5.4 cm ± 1.2 cm, the outcome of pregnancy was CS. The cutoff value was 4.2 cm.
However, Gilboa et al. [24] studied 65 ladies showing a prolonged second stage of labor and found no any statistically significant correlation between FHPD and the mode of labor. These previous results did not match with the current study. However, our results are concordant with Kalache et al. [1] and Barbera et al. [10, 11], who found that the less the FHPD, the more likely the labor will be spontaneous vaginal delivery.
Fetal head perineal angle by TPUS is a reflection of the dynamics of head progression [25, 26]. Kalache et al. [1] confirmed that “angle of progression” is a simple US parameter using two objective US landmarks: the symphysis pubis and the leading bony edge of the fetal skull avoiding the ischial spines which are used during digital vaginal examination. They found a high predictive value of a wide “angle of progression” and spontaneous vaginal delivery.
Amin et al. [14] stated that “angle of progression” ≥ 120° was correlated with an 85.5% probability of vaginal delivery. Malik and Singh [15] estimated an angle of progression ≥ 116° as a predictive of vaginal delivery in the late first and second stages. Barbera et al. [10, 11] and Kalache et al. [1] also noticed a continuous increase in the “angle of progression” during the second stage in all the vaginal deliveries. They claimed that all women with an angle of progression > 120° delivered spontaneously. The angle of progression ≤ 108° was used as a cutoff value for patients who are in need for CS. This agreed with the current study which shows a significant correlation between a large angle of fetal head descent and the success of vaginal delivery.
Torkildsen et al. [23] reported that the predictive value of vaginal delivery was 81% and 76% for FHPD and angle of progression, respectively. They used 110 degrees as a cutoff value for the angle of progression; 87% delivered vaginally (sensitivity 56%, specificity 75%, PPV 87%, NPV 37%, positive LR of 2.2, and negative LR of 0.6). However, in our study, using cutoff value for angle of progression of 115 degrees; 91% delivered vaginally (sensitivity 93%, specificity 84%, PPV 87%, NPV 91%, positive LR 6.06, and negative LR 0.08).
According to Torkildsen et al., [23] by using a cutoff value of 4 cm for the FHPD, 93% delivered vaginally (sensitivity 62%, specificity 85%, PPV 93%, NPV 43%, positive LR of 4.2, and negative LR of 0.4). Similar, but with a lower predictivity, to our study using 4.2 cm as a cutoff value, 84% delivered vaginally (sensitivity 80%, specificity 84%, PPV 85%, NPV 78%, positive LR 5.2, and negative LR 0.24).
Fetal complications of prolonged labor—including caput succedaneum and molding—were diagnosed easily by TPUS, which is one of the strongest points of our study. Thirty-five percent of cases developed molding, and 28% of cases developed caput succedaneum, which was confirmed clinically after delivery.
This study has several limitations as well; the small sample size and lack of control of some confounding factors like the different obstetricians managing the labor and other confounding factors regarding the general state of the pregnant ladies and fetal parameters.