Intrapartum ultrasound
2007; Wiley; Volume: 30; Issue: 2 Linguagem: Português
10.1002/uog.4096
ISSN1469-0705
Autores Tópico(s)Maternal and fetal healthcare
ResumoUltrasound in Obstetrics & GynecologyVolume 30, Issue 2 p. 123-139 EditorialFree Access Intrapartum ultrasound D. M. Sherer, Corresponding Author D. M. Sherer dmsherer@aol.com Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Box 24, Brooklyn, NY 11203, USADivision of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Box 24, Brooklyn, NY 11203, USASearch for more papers by this author D. M. Sherer, Corresponding Author D. M. Sherer dmsherer@aol.com Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Box 24, Brooklyn, NY 11203, USADivision of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Box 24, Brooklyn, NY 11203, USASearch for more papers by this author First published: 23 July 2007 https://doi.org/10.1002/uog.4096Citations: 44AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat INTRODUCTION Throughout the last decade, robust, compact, mobile, high-resolution real-time ultrasound machines, often capable of duplex and color Doppler imaging and designed to enable service at the patient's bedside, have become increasingly available. As a result, most labor and delivery services can (and many do) provide continuous 24-h, on-site, on-demand, ultrasound capability. This technology has become so user friendly that non-ultrasound-trained professionals, including attending physicians, residents, nurses and physician assistants, routinely provide high-quality basic ultrasound services. Undoubtedly, the availability of immediate and convenient diagnostic and intervention-guided ultrasound support has enhanced medical care in the demanding labor and delivery environment. Although, overall, most obstetric ultrasound examinations will clearly remain in the antepartum arena, it is essential to understand current applications of ultrasound in the setting of labor and delivery. Accordingly, in 2002 the Journal dedicated an opinion article to the role of ultrasound scanning on the labor ward1. A growing body of knowledge is accumulating regarding true intrapartum ultrasound, a relatively new application of ultrasound. Accordingly, currently available data on intrapartum ultrasound assessment of active labor, limitations, recent developments and future possibilities, as well as other pertinent intrapartum applications of ultrasound examination as an adjunct to the overall management of the patient in labor (intrapartum ultrasound-assisted identification of the epidural space, maternal postvoid residual and total bladder volume measurements) and investigative data (uterine contractions, intrapartum myometrial contractility, the third stage of labor, intrapartum maternal and fetal hemodynamics and fetal behavior), are presented in this Editorial. Current applications of ultrasound imaging in labor and delivery By virtue of its safety and non-invasive nature, given appropriate circumstances, intrapartum ultrasound examination is applicable for most (if not all) indications currently practiced in the antepartum arena. These include fetal biometry and weight assessment, fetal presentation(s), assessment of the breech-presenting fetus, multiple gestations, placental positioning, anatomy, preinduction cervical assessment, preinduction transvaginal ultrasonographic prediction of successful induction or operative delivery, fetal testing (amniotic fluid volume, biophysical profile, Doppler flow velocimetry), assessment of the size and location of (possibly obstructing) uterine leiomyomata, assessment of amniotic fluid volume following amnioinfusion, and procedure guidance (transabdominal and transvaginal cephalocentesis, thoracocentesis, drainage of fluid-filled fetal tumors, external cephalic and internal podalic version, manual removal of the placenta and placement of central intravenous lines)2-36. Similarly, intrapartum ultrasound has been used in the diagnosis of esoteric conditions such as uterine rupture and vasa previa37-39. Overall, few data are available regarding utilization of ultrasound imaging in labor and delivery services, which may vary according to various practices and clinical settings (academic teaching institutions, community hospitals, birthing centers). Indications for ultrasound examination in labor and delivery suites in an active academic teaching institution in the USA were assessed over a 4-month period40. Not surprisingly, this study demonstrated that 68% of all ultrasound examinations in this service were performed on non-laboring patients (who were ultimately discharged undelivered following that examination), compared with 32% of studies performed on patients in labor. Of all patients delivered during the study period only 14.5% underwent true intrapartum ultrasound assessment, whereas 30.7% of all patients discharged undelivered had ultrasound assessments40. As might have been expected, the mean gestational age of patients at the time of assessment was significantly higher among laboring than non-laboring patients (mean ± SD 37.32 ± 4.23 weeks' gestation vs. 35.74 ± 5.67 weeks' gestation, respectively; P < 0.050). Main indications for ultrasound scan in patients in labor were: fetal presentation in patients with spontaneous rupture of membranes (34.4%), confirmation of cephalic presentation (20.3%), preterm labor (12%), multiple gestation (7.3%) and malpresentation (7.3%). In contrast, main indications for examinations of those not in labor related to various tests of fetal well-being: amniotic fluid index (AFI) (15.8%), spontaneous rupture of membranes (15.6%), postdates (9.8%), placental location (9.6%) and decreased fetal movements (9.3%)40. Intrapartum ultrasound As a direct result of the irregular shape of the maternal pelvis and the relatively large dimensions of the fetal head at term, not all diameters of the latter can necessarily traverse all diameters of the maternal pelvis. Accommodation or adaptation of portions of the fetal head to various segments of the pelvis is therefore required for vaginal delivery. Such positional changes constitute the cardinal movements of labor, and include engagement, descent, flexion, internal rotation, extension, external rotation and expulsion41, 42. Additional adaptive measures include intrapartum molding of the fetal head (changes in fetal head shape resulting from external compressive forces acting upon the fetal cranial vault)41, 42. Molding of the fetal head and caput succedaneum Ultrasonographic depictions of various adaptations of the fetal head to the maternal pelvis, for example molding of the fetal head, have been reported43. Similarly, indentation or compression of the fetal parietal bone may be depicted in association with obstructed labor or with the external pressure of uterine leiomyomata44. The occurrence of both caput succedaneum (swelling over the most dependent area of the fetal scalp immediately over the cervical os in prolonged labors before complete cervical dilatation) and cephalhematomas (representing subperiosteal hemorrhage) have been reported not only during but also before labor45-49. Of interest, among non-laboring patients these appear more common in the setting of oligohydramnios or premature rupture of the membranes and possibly reflect the result of Braxton-Hicks contractions. The clinical significance of the presence of these ultrasonographically demonstrated changes is currently unclear. Engagement, descent, flexion Engagement of the fetal head in the maternal pelvis pertains to the biparietal diameter (BPD; the greatest transverse diameter of the fetal head) having successfully traversed the anteroposterior diameter of the pelvic inlet41, 42. Both flexion and descent of the fetal head contribute to successful engagement, which may occur during the last weeks of pregnancy or (as recent evidence suggests also among nulliparous patients) only after labor commences50-52. Ultrasound imaging may be utilized with relative ease to depict flexion of the fetal head. This may be noted directly while tracking the fetal spine in a sagittal plane towards the fetal head. Of note, various degrees of deflexion or extension of the fetal head at the initiation of labor have been associated with lack of engagement44, 53. At times this may reflect various mechanical problems, such as an obstructing leiomyoma of the lower uterine segment or the occurrence of a face presentation (acute hyperextension), which may prevent successful engagement. For example, in an unusual case at the onset of labor, location of the BPD in a plane perpendicular to the maternal pelvic inlet was observed (with sagittal rather than transverse suprapubic placement of the transabdominal transducer)44. This ultrasonic finding confirmed significant deflexion of the fetal head resulting from a leiomyoma of the lower uterine segment compressing the parietal bone and obstructing labor44. Varying degrees of deflexion of the fetal head may be noted as a result of movement of the depicted BPD from an imaginary line parallel to the pelvic inlet, to any angle up to 90°, the latter representing an acutely hyperextended fetal head–face presentation53. Clinically, engagement of the fetal head can be ascertained by either abdominal or transvaginal digital examination41, 42. Each of these assesses fetal head engagement in an indirect fashion. At abdominal examination, engagement is considered to have occurred when only two-fifths of the fetal head is palpable42. Alternatively, when the BPD of a term-sized fetus has descended through the pelvic inlet, the examining fingers cannot reach the lowermost part of the fetal head. As a result, when pushed downwards over the abdomen, the examining fingers will slide over the portion of the fetal head that is proximal to the BPD and diverge49. Conversely, if the fetal head is not engaged, the examining fingers can easily palpate the lower part of the fetal head and will converge (the fourth Leopold maneuver)54. Engagement of the fetal head is usually considered to have (but not always) occurred at transvaginal assessment when the presenting part (usually the parietal bone) is positioned at maternal ischial spine station 0. This pelvic station is considered the key for fetal head engagement assessment purposes, given that the average distance between the plane of the pelvic inlet and the ischial spines is approximately 5 cm, whereas the distance between the BPD plane and the unmolded fetal occiput averages 3–4 cm54. Under normal circumstances, the fetal head cannot reach ischial spine station 0 unless the BPD has traversed the pelvic inlet, or there has been significant elongation of the fetal head resulting from molding and formation of caput succedaneum54. Although engagement of the fetal head is usually considered following findings at physical examination, in the past, plain radiography and, nowadays, computed tomography (CT) scan and magnetic resonance imaging (MRI), may provide imaging confirmation of engagement. Neither of the latter modalities is available at the bedside in labor and delivery suites, and currently they are seldom employed in the clinical management of labor in women with cephalic-presenting fetuses. In contrast, it appears that intrapartum ultrasound examination may be utilized to assess engagement of the fetal head. Various sonographic modalities have been employed in the intrapartum determination of fetal head engagement. In a study of 222 consecutive patients in labor (≥ 37 weeks' gestation with normal singleton cephalic-presenting fetuses with cervical dilatation ≥ 1 cm, with either ruptured or intact membranes), transverse suprapubic sonography was compared with transvaginal digital examination as the 'gold standard' technique of intrapartum assessment of fetal head engagment55. In this study the imaginary line representing the pelvic inlet was demarcated by directing the transverse suprapubically positioned transabdominal transducer towards the maternal sacral promontory. Engagement of the fetal head (or the lack thereof) was ascertained according to whether or not the fetal BPD was depicted below or above the pelvic inlet, respectively. Overall, transvaginal digital examinations were consistent with ultrasound determinations with a raw agreement rate of 85.6% (95% CI, 80.8–90.3) (κ = 69.5% (95% CI, 59.4–73.9); P < 0.001). When stratified according to parity, the agreement for nulliparous patients was 81.5% (95% CI, 73.4–88.0) (κ = 60.7% (95% CI, 45.9–64.1); P < 0.001) and for parous patients it was 90.3% (95% CI, 84.1–95.9) (κ = 80.4% (95% CI, 63.0–87.5); P < 0.001). Maternal age, gravidity, maternal body mass index, gestational age, cervical dilatation, effacement, membrane status (ruptured or intact), ischial spine station of the fetal head, fetal head position at ultrasound assessment, birth weight, mode of delivery and examiner experience did not affect the rate of agreement55. Dietz and Lanzarone subsequently utilized mid-sagittal translabial ultrasonography to determine fetal head engagement in comparison to transvaginal digital examination56. Sonographic assessment of fetal head engagement was performed by two novel methods. In the first, a line through the inferoposterior symphyseal margin, parallel to the main transducer axis, was employed as reference. The minimum distance between this line and the presenting part depicted by translabial imaging was measured in millimeters. In the second method, the line of reference was a line vertical to the central axis of the symphysis pubis placed through the symphyseal margin, similar to an alternative method proposed for pelvic floor imaging. Again, head engagement was defined as the minimum distance between the presenting part and this line. In this study, translabial ultrasound depiction of head engagement correlated strongly with abdominal palpation, Bishop score and digital transvaginal assessment56. Intraclass correlation coefficients for the two translabial ultrasound methods were 0.75 and 0.92, respectively, confirming excellent interobserver agreement. Reproducibility was higher for the method using the central symphyseal axis as a reference56. Internal rotation (fetal head position) Established data regarding intrapartum fetal head position assessment are based on the time-honored classical röentgenological study of Caldwell and Moloy57 and a subsequent study of Calkins58, both published in the 1930s. Recent evidence suggests that intrapartum ultrasonographic assessment of fetal head position is possible, and is probably more precise than is digital transvaginal assessment. In 1989 Rayburn et al. studied 86 women exhibiting an arrest of cervical dilatation (> 7 cm)59. These authors noted that, although occiput transverse positions were diagnosed accurately by digital transvaginal assessment, the distinction between persistent occiput posterior or anterior positions was often inexact by palpation alone. Combined clinical and ultrasound techniques resulted in improved determinations of fetal head position. Interestingly, these authors experienced difficulty in determining between left or right occipitotransverse positions, possibly reflecting earlier ultrasound resolution59. Gardberg et al., using intrapartum ultrasound examination, determined that in most cases persistent occipitoposterior position resulted from an intrapartum malrotation60. Only 32% of persistent occipitoposterior positioned fetuses exhibited an absence of rotation from an initial occipitoposterior position documented at the onset of labor60. In a subsequent study, Akmal et al. showed by intrapartum ultrasound imaging that the majority of occipitoposterior positions during labor rotated to the anterior position even at 10 cm of cervical dilatation61. Interestingly, these authors noted that most occiput posterior positions at delivery are a consequence of persistence of this position rather than malrotation from an initial occiput anterior or transverse position, in contrast to the findings of Gardberg et al.60. Lieberman et al. conducted a prospective cohort study of 1562 women to evaluate changes in fetal head position in labor. Serial ultrasound examinations were performed at enrollment, epidural administration and during advanced labor (> 8 cm)62. It was found that, of fetuses that were occipitoposterior in advanced labor, only 20.7% were occipitoposterior at delivery. Changes in fetal head position were common, and 36% of women had an occipitoposteriorally positioned fetus on at least one ultrasound examination. These authors also noted that, although at enrollment occipitoposterior positioning of the fetus was no more common in women who received epidural anesthesia than in those who did not (23.4% vs. 26%, respectively), women who had epidural anesthesia had a higher incidence of fetuses in the occipitoposterior position at delivery (12.9% vs. 3.3%, respectively)62. In two sequential studies, transvaginal digital examinations were compared with transabdominal ultrasound assessments of fetal head position during the first and second stages of labor63, 64. All transabdominal suprapubic ultrasound examinations performed in the active stage of labor (n = 102) and the second stage of labor (n = 112) yielded precise determinations, undoubtedly reflecting markedly improved ultrasound resolution since earlier studies. Regarding the active stage of labor, transvaginal digital examinations were consistent with ultrasound assessments in only 24% of patients (n = 24; 95% CI, 16–33; P = 0.002, κ = 0.12)63. Logistic regression revealed that cervical effacement (P = 0.03) and ischial spine station (P = 0.01) significantly affected the accuracy of transvaginal digital examination. Parity, gestational age, epidural anesthesia, cervical dilatation, birth weight and examiner experience did not significantly affect the accuracy of the examination. The accuracy of the transvaginal digital examinations (n = 48) was increased to 47% (95% CI, 37–57; κ = 0.16) when fetal head position at transvaginal digital examination was recorded as correct if reported within ± 45° of that determined by ultrasound assessment63. The rate of agreement (within ± 45° analysis) between the two assessment methods for attending physicians and residents was 58% and 33%, respectively (P = 0.02). In summary, these data demonstrate an overall high rate of error (76%) in transvaginal digital compared with ultrasound determinations of fetal head positioning during active labor63. Throughout the study the structure most consistently defining intracranial fetal head position was the thalamus63. Attending physicians exhibited an almost twofold higher success rate than did residents in depicting correct fetal head positioning by physical examination. This study demonstrated that intrapartum ultrasound imaging increases the accuracy of fetal head position assessment during active labor and may serve as an educational tool for physicians in training63. Interestingly, the original observation of Caldwell et al. (derived from radiographic studies) that the fetal head usually engages in the occipitotransverse position, more commonly in the left than right positions (58.5% vs. 40.5%)57, was strikingly confirmed by intrapartum ultrasound examination in the study by Sherer et al. (56.8% vs. 34.5%)63. Regarding the second stage of labor, absolute error of transvaginal digital examination was recorded in 65% (95% CI, 56–74) of patients64. Parity, pelvic station, combined spinal epidural anesthesia, length of first or second stages of labor, use of oxytocin augmentation, gestational age, mode of delivery, birth weight and examiner experience did not significantly affect the accuracy. Stratification, with the transvaginal digital examination recorded as correct if giving a value within ± 45° of the ultrasound assessment, reduced the error of transvaginal digital examination to 39% (95% CI, 30–49). Independent variables again did not affect the accuracy of this assessment modality. Rates of agreement between the two methods for attending physicians compared with residents were not significantly different. The overall levels of agreement were 40% (95% CI, 26–55) and 68% (95% CI, 53–80) (κ = 0.25 and κ = 0.30) for the absolute agreement and ± 45° assessment modalities, respectively, for attending physicians, and 31% (95% CI, 20–44) and 55% (95% CI, 42–68) (κ = 0.14 and κ = 0.12) for senior residents64. These data demonstrated a high rate of error (65%) in transvaginal digital compared with ultrasound determinations of fetal head positioning during the second stage of labor and again indicate that intrapartum ultrasound examination increases the accuracy of fetal head position assessment during the second stage of labor64. In contrast to the first stage of labor, the performance of senior residents in transvaginal examinations did not differ significantly from that of attending physicians. This was considered to reflect the fact that transvaginal digital examination is less challenging during the second stage of labor when the cervix is fully dilated and a larger portion of the fetal head is palpable. Interestingly, similar to the data of Calkins, intrapartum ultrasound-derived data confirmed that 53.6% of fetuses were in an occipitoanterior-directed position (occipitoanterior, 14.3%; left occipitoanterior, 28.6%; right occipitoanterior, 10.7%), and also confirmed the finding of Caldwell et al.57 that left-orientated occiput positions are more common than right64. Souka et al., in a longitudinal study of women in the first and second stages of labor, either normal or obstructed, assessed the feasibility of intrapartum transabdominal ultrasound imaging in determining fetal head position in comparison with transvaginal digital examination65. These authors noted that assessment of the fetal head position was not possible by digital assessment in 60.7% (122/201) of cases in the first stage of labor and 30.8% (41/133) in the second stage. In cases in which digital assessment was available, the correlation with ultrasound assessment was average in the first stage of labor (κ = 0.59) and good in the second stage (κ = 0.77). Overall, fetal head position assessment by digital examination was accurate in 31.3% of cases in the first stage and 65.7% in the second stage65. These authors concluded that digital examination of the fetal head position is less accurate than is intrapartum ultrasound assessment, in particular in cases of obstructed labor, when medical intervention is more likely to be needed65. Other studies have confirmed that ultrasound imaging is more accurate than is digital vaginal examination in determining the precise position of the fetal head in the second stage of labor. Dupuis et al. studied fetal head position in 110 patients in the second stage of labor, comparing transabdominal ultrasound with digital transvaginal examination66. These authors noted that ultrasonographic and clinical examination produced results that differed significantly, by more than 45°, in 20% of cases66. This rate reached 50% for fetuses in the occipitoposterior and occipitotransverse positions. Not surprisingly, these authors noted that the presence of caput succedaneum tended to diminish the accuracy of clinical examination (P = 0.09). They concluded that transabdominal ultrasonography is a simple, quick and efficient way of increasing the accuracy of the assessment of fetal head position during labor66. Kreiser et al., investigating whether ultrasonography is superior to transvaginal digital examination in determining fetal occiput position during the second stage of labor, used transperineal ultrasound in addition to transabdominal ultrasound imaging67. Analysis of prospective data from a cohort of 44 parturients showed that the error rate in detecting fetal occiput position was significantly lower when combined abdominal and transperineal ultrasound imaging was used compared with transvaginal digital vaginal examination (6.8% vs. 29.6%, respectively; P = 0.011). Parity, maternal body mass index and fetal weight did not influence the error rate67. Using similar methodology Chou et al. studied 88 patients in the second stage of labor, comparing fetal head positions suggested by clinical examination and intrapartum ultrasound imaging. Predictions were compared with the actual position at spontaneous vaginal delivery or at Cesarean delivery68. Digital transvaginal examination correctly determined fetal occiput position in 71.6% of cases, whereas intrapartum ultrasound examination was correct in 92% (P = 0.018)68. Akmal et al. investigated the accuracy of intrapartum transvaginal digital examination in defining the position of the fetal head in 496 women with singleton pregnancies at term69. Digital examination was considered correct if within 45° of the ultrasound finding69. Digital examination failed to define the fetal head position in 166 (33.5%) cases. In 330 cases in which the position was determined, the digital and sonographically derived positions were in agreement in only 163 (49.4%) cases. The rate of correct identification of the fetal head position by digital examination increased with cervical dilatation (from 20.5% at 3–4 cm to 44.2% at 8–10 cm)69. The authors of this relatively large study confirmed earlier reports that routine intrapartum digital vaginal examination fails to identify correct fetal head position in the majority of cases69. Subsequently, Akmal et al., in a study of 60 women with singleton pregnancies at term with cervical dilatation ranging from 3 to 10 cm, determined that the interobserver agreement regarding sonographic determination of fetal head position during labor is within 15° in nearly 90% of cases and 30° in all cases70. Finally, Akmal et al. used fetal head position in addition to traditional maternal, fetal and labor-related characteristics in predicting the likelihood of Cesarean delivery71. Among 601 patients in the early stages of labor at term with singleton cephalically presenting fetuses, 514 (86%) vaginal and 87 (14%) Cesarean deliveries were recorded. The fetal occiput position was posterior in 209 (35%) cases. In this group the incidence of Cesarean delivery was 19% (40 cases), compared with 11% (47/392) in the non-occipitoposterior group. Multiple regression analysis revealed that significant independent predictors of Cesarean delivery were maternal age (odds ratio (OR) 1.1; 95% CI, 1.0–1.2), African–Caribbean origin (OR 2.4; 95% CI, 1.2–4.6), height (OR 0.93; 95% CI, 0.89–0.97), parity (OR 0.2; 95% CI, 0.1–0.4), type of labor (OR 2.2; 95% CI, 1.3–3.8), gestation (OR 0.2; 95% CI, 1.1–1.7), fetal head descent (OR 0.6; 95% CI, 0.4–0.9), occiput posterior position (OR 2.2; 95% CI, 1.3–3.7) and male sex (OR 2.0; 95% CI, 1.2–3.5)71. Assessment of fetal head position before instrumental delivery In cases of arrest of the second stage of labor, molding of the fetal head and the presence of caput succedaneum may hinder clinical assessment of precise fetal head positioning. Indeed, Akmal et al. used intrapartum ultrasound imaging to demonstrate that, in this clinical scenario before instrumental delivery, digital transvaginal examination fails to define the position of the fetal head in approximately one-quarter of cases72. Intrapartum ultrasound imaging may assist in correct placement of the vacuum extractor cup or alternatively forceps blades, possibly enhancing the success of instrumental delivery1, 64, 72. Henrich et al. recently investigated 20 patients in spontaneous labor at term who had clinical indications for vacuum-assisted delivery (failure to progress or non-reassuring fetal heart rate tracing)73. Intrapartum translabial ultrasound imaging was conducted before vacuum extraction. Parameters assessed at intrapartum ultrasound examination included descent of the fetal head during maternal pushing efforts, widest part below the infrapubic line and head direction (up/horizontal/down)73. Four of these patients experienced a 'difficult' and one a failed vacuum delivery. When there was no objective descent of the fetal head at intrapartum ultrasonography during maternal pushing efforts, vacuum delivery was either difficult (two cases) or failed (one case). Of note, large caput succedaneum was noted before difficult and failed vacuum extractions73. Although conclusive and irrefutable data are not available, it appears likely that intrapartum ultrasound depiction of the precise fetal head position may allow safer operative vaginal deliveries with improved outcome. Cervical effacement Few data are available relating to ultrasonography of cervical changes (effacement and dilatation) during labor. Zilianti et al. monitored the process of effacement of the uterine cervix of 86 patients in early labor with normal-term pregnancies and intact membranes by applying transperineal sonography in both longitudinal and transverse planes74. These authors used 3.5- and 5-MHz sector and 3.5-MHz convex tranducers, preferring the latter because it provided a wider panoramic view. Cervical effacement was depicted at longitudinal scanning, whereas initial di
Referência(s)