Although most obstetric ultrasound examinations are performed to make antenatal diagnoses of fetal abnormalities, the use of ultrasound for women in labor is increasing as a result of accumulating evidence of its usefulness for objectively evaluating the progress of labor.

When progress failure or fetal distress occurs, obstetricians should choose between an operative vaginal delivery or a cesarean section. For the fetus successful instrumental delivery is safer than emergency cesarean delivery. Because the fetal head is impacted deeply in the maternal pelvis, second-stage cesarean section is in association with increased maternal risks such as major hemorrhage, bladder injury, and extension tears of the uterine angle resulting in broad ligament hematoma. Furthermore, cesarean section after failed vacuum extraction is also associated with an increased risk of fetal trauma. The incidence of intracranial hemorrhage with cesarean section following failed operative vaginal delivery is 1/334 compared to 1/860 with successful vacuum delivery [1]. Therefore, it remains to be determined if intrapartum ultrasound can be used to diagnose the presence of abnormal labor and predict the possibility of successful instrumental delivery, consequently improving maternal and fetal outcomes.

As determinants of normal vs. abnormal labor, the specific targets of intrapartum ultrasound are the engagement of the fetal head into the maternal pelvis, fetal head station, the detection of occiput posterior position during labor, and placental separation. In this review, we discuss the usefulness of intrapartum ultrasound for the evaluation of labor progress and predicting successful operative vaginal delivery.

During labor, the fetus exhibits the cardinal movements of labor, which include engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. Regarding cardinal movement, ultrasound examination may provide more accurate information about fetal position than digital examination.

In a study of 496 single pregnancies in labor at term, digital examination failed to detect the fetal head position in 166 cases (33.5%); moreover, the digital and sonographic findings were concordant in only 163 cases [2]. Similarly, another study of 112 patients in the second-stage of labor at term with normal single cephalic-presenting fetuses and membrane rupture demonstrated a high rate of error (65%) of the vaginal digital assessment of fetal head position compared to ultrasound assessment as the gold standard [3]. Interestingly, in that study, there was no difference in the technique of senior residents and attending physicians. However, during active labor, attending physicians were almost twice as successful at measuring the correct fetal head position by physical examination. Nevertheless, the discordance rate between vaginal examination and ultrasound assessment was also high (76%) during active labor [4].

The main reason for using sonography to define fetal position is to diagnose persistent occiput posterior position (POPP). POPP is a well-known cause of abnormal labor and occurs in approximately 5% of deliveries and 20% at labor onset. POPP is associated with approximately 4- and 13-fold higher rates of operative vaginal and cesarean deliveries, respectively [5,6]. Occiput posterior position during labor mostly changes to the anterior position even at full cervical dilatation. Nevertheless, most occiput posterior positions at delivery are the initial occiput posterior position rather than misrotation from an original occiput anterior or transverse position [5,6]. Several recent studies support this notion of POPP as the main cause of occiput posterior delivery, although the concept of malrotation of the initial occiput anterior or transverse position for POPP prevailed even until approximately a decade ago [5,7,8].

Blasi et al. [7] suggest that the positions of the head and spine during the second stage of labor could be practical indicators for predicting the occiput posterior position at delivery. In their prospective cohort study, 100 singleton pregnant women underwent intrapartum ultrasound during the first and second stages of labor, and the positions of the fetal head and spine were defined. The rate of occiput posterior position during the first stage of labor was 51%, but the majority of these cases rotated to an anterior position before delivery. There were 6 cases of occiput posterior position at delivery; all of these were among the 23 fetuses in an occiput posterior position during the second stage of labor. With occiput posterior and spine anterior position on ultrasound, none of the babies was born in the occiput posterior position. On the other hand, the fetuses presenting occiput posterior position at delivery also had a posterior spine position during the second stage of labor. If the results of this study are confirmed with larger sample sizes, they could be very helpful for managing POPP pregnancies.

Furthermore, digital pelvic examination is inferior to ultrasound for assessing the fetal head transverse position during labor. In particular, the caput succedaneum related to the deep transverse position decreases the diagnostic accuracy of vaginal digital examination. The transverse position of the fetal head can interrupt fetal descent through asynclitism. A recent study by Malvasi et al. [9] shows that "squint sign" and "sunset of thalamus and cerebellum signs" are 2 simple ultrasonographic signs for anterior and posterior asynclitism, respectively. The most frequent transverse position was the left one, while the most frequent asynclitism was the anterior one. The transverse head positioning with anterior or posterior asynclitism is unlikely to be promoted by drug- or performance-related mechanisms but should rather be a consequence of cephalopelvic disproportion. Furthermore, epidural analgesia does not raise the rate of dystocic labor [10].

According to the classification of the American College of Obstetricians and Gynecologists, station divides the pelvis above and below the ischial spines into fifths at 1-cm intervals. Zero station means that the lowermost fetal presenting part is at the level of the spines. Meanwhile, station +5 is the status of the fetal head being visible at the introitus. However, the digital examination of fetal head station is unreliable. Dupuis et al. [11] investigated the reliability of digital examination of fetal head station assessed by 32 residents and 25 attending physicians using a newly designed birth simulator mannequin. The error rates of residents and attending physicians were 50% to 88% and 36% to 80%, respectively. Furthermore, caput succedaneum, which forms during labor, can also hinder accurate digital examination of fetal station. Therefore, objective measurements for the engagement and station in labor are required.

Currently used ultrasonographic markers to measure the fetal station during labor include head-perineum distance, angle of progression, fetal head-symphysis distance, intrapartum translabial ultrasound station, and fetal direction.

The cervical change, called ripening, occurs in preparation for labor and delivery; it comprises dilatation, shortening, and softening and is essential for labor progress. Regarding labor, ultrasonography of the cervix usually focuses on length. However, many investigators have recently paid attention to softening and stiffness as well. Elastography is a tool for evaluating stiffness on the basis of the estimated displacement of moving tissues while oscillatory pressure is applied. The background physical properties are stress, strain, and deformation. The clinical use of ultrasound elastography for characterizing tissue hardness is increasing because it is better, safer, and easier than other methods. For example, breast elastography is a useful complementary tool for undetermined breast lesions or cystic lesions [25]. Moreover, ultrasound elastography has been used successfully in the diagnosis and classification of breast cancer [26]. Malignant masses evaluated by elastography are usually heterogeneous and irregular; furthermore, they are larger on elastography than on grayscale imaging. Ultrasound elastography was recently introduced in the field of obstetrics. Hernandez-Andrade et al. [27] evaluated cervical stiffness during pregnancy by estimating the average tissue displacement (i.e., strain) within a defined area of interest. A total of 1,557 strains were checked in 262 patients at 8 to 40 weeks of gestation. Their results show the strain is greater in the endocervix than the entire cervix, in the external os than the internal os, and in parous women than nulliparous women. A new method using the quantification of cervical elastographic colors has been introduced as well [27]. However, Molina et al. [28] indicate that this does not mean the measurements of rate-of-change in tissue placement is equal to histological changes in cervical ripening, although this point remains to be clarified. Hwang et al. [29] recently evaluated the ability of cervical elastography for predicting successful labor induction in nulliparous women at term. In their study of 145 women, receiver operating characteristic curves and the areas under the curve for the combination of cervical length, cervical area, mean elastographic index, and cervical hard area were calculated. The combined data of cervical length and elastographic data were more predictive of successful labor induction than any other combination.

Cervical elastography is limited because of the absence of reference tissue in that anatomical area, in contrast to the breasts where adipose tissue is a reliable one [30]. Therefore, it has been difficult to quantitatively compare cervical stiffness among women. Hee et al. [31] report quantitative elastography using Young's modulus (N/mm²), which is the approximate tissue stiffness based on reference caps made of silicone and oil. The Young's modulus of the anterior cervical lip is 0.08 and 0.03 N/mm² in mid- and full-term pregnant women, respectively. The same group recently evaluated elastographic data using Young's modulus for the prediction of prolonged cervical dilation time. They found that the Young's modulus is associated with cervical dilation time during active labor (P<0.01) and predicts increased cervical dilation time (>330 min) with a sensitivity of 74% and specificity of 69% [32].

Considering the important role of the cervix in normal labor and the technology of elastography to reflect structural changes such as tissue hydration, collagen structure, and tissue elasticity, cervical elastography is promising for predicting successful labor induction [33].

Abnormal placental separation contributes to hemorrhagic morbidity and mortality in pregnancy. The placental separation has been evaluated using ultrasound. Ultrasonographic findings indicate the placental separation is divided into 3 (or 4) phases: the latent, (contraction), detachment, and expulsion phases. The latent phase is the interval between fetal delivery and the beginning of placental separation. The contraction phase is characterized by thickening the placenta site wall. The detachment phase involves monophasic or multiphasic separation of the placenta. Finally, the expulsion phase is the interval between completed separation and vaginal delivery of the placenta. The characteristic finding of normal placental separation is the cessation of blood flow between myometrium and basal placenta after the birth of the fetus; continuous flow on color Doppler between those structures may indicate placenta accreta [34].

The most important issue regarding placental separation is how the separation proceeds. Recent data show that the placental separation is usually an multiphasic phenomenon in sequence that starts mostly from the lower pole of the placenta and spreads upwards. The fundal placenta separates first at the poles and last at the fundal part [35]. Understanding the mechanism of placental separation may help the management of complications in the third stage of labor; ultrasound will undoubtedly play a central role in elucidating this mechanism.

Intrapartum ultrasound not only provides objective and quantitative data in labor, thus expanding knowledge about the pathophysiology of abnormal labor, but also improves obstetric outcomes of both the mother and fetus as a supplementary tool for active management. With increasing resolution and easier accessibility, ultrasound is receiving more and more attention during labor. Practicing obstetricians can use intrapartum ultrasonographic findings on fetal head status (i.e., engagement, station, and position), cervical status (i.e., shortening, dilatation, and softening), and placental separation to make more reliable clinical decisions.