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Human birth observed in real-time open magnetic resonance imaging

Published:January 16, 2012DOI:https://doi.org/10.1016/j.ajog.2012.01.011

      Objective

      Knowledge about the mechanism of labor is based on assumptions and radiographic studies performed decades ago. The goal of this study was to describe the relationship between the fetus and the pelvis as the fetus travels through the birth canal, using an open magnetic resonance imaging (MRI) scanner.

      Study Design

      The design of the study used a real-time MRI series during delivery of the fetal head.

      Results

      Delivery occurred by progressive head extension. However, extension was a very late movement that was observed when the occiput was in close contact with the inferior margin of the symphysis pubis, occurring simultaneously with gliding downward of the fetal head.

      Conclusion

      This observational study shows, for the first time, that birth can be analyzed with real-time MRI. MRI technology allows assessment of maternal and fetal anatomy during labor and delivery.

      Key words

      Thorough understanding of maternal and fetal anatomy and physiology is essential for proper management of labor and delivery. The mechanism of labor is generally understood as the movements of the fetus in relation to the bony structures and soft tissues of the birth canal during labor. More than 1000 radiographic examinations of the pelvis and fetal head before, during, and after labor were performed in early studies to better understand the mechanism of birth.
      • Borell U.
      • Fernström I.
      The mechanism of labour.
      • Caldwell W.
      • Moloy H.
      Anatomical variations in the female pelvis and their effect in labor with a suggested classification.
      • Caldwell W.
      • Moloy H.
      • D'Esopo D.
      Further studies on the mechanism of labor.
      However, prenatal x-ray exposure has been associated with an increased risk of childhood cancer in case-control studies.
      • Stewart A.
      • Webb J.
      • Giles B.
      • Hewitt D.
      Malignant disease in childhood and diagnostic irradiation in utero.
      • Stewart A.
      • Webb J.
      • Hewitt D.
      A survey of childhood malignancies.
      • Bithell J.F.
      • Stewart A.M.
      Pre-natal irradiation and childhood malignancy: a review of British data from the Oxford Survey.
      Click Supplementary Content under the article title in the online Table of Contents
      Nonionizing radiation is preferable in pregnant women. Numerous trials of magnetic resonance imaging (MRI) have not revealed any experimental or clinical evidence of fetal harm
      • Clements H.
      • Duncan K.R.
      • Fielding K.
      • Gowland P.A.
      • Johnson I.R.
      • Baker P.N.
      Infants exposed to MRI in utero have a normal paediatric assessment at 9 months of age.
      • Kok R.D.
      • de Vries M.M.
      • Heerschap A.
      • van den Berg P.P.
      Absence of harmful effects of magnetic resonance exposure at 1.5 T in utero during the third trimester of pregnancy: a follow-up study.
      • Baker P.N.
      • Johnson I.R.
      • Harvey P.R.
      • Gowland P.A.
      • Mansfield P.
      A three-year follow-up of children imaged in utero with echoplanar magnetic resonance.
      • Myers C.
      • Duncan K.R.
      • Gowland P.A.
      • Johnson I.R.
      • Baker P.N.
      Failure to detect intrauterine growth restriction following in utero exposure to MRI.
      ; thus, MRI is considered safe for the mother and fetus.
      • Chen M.M.
      • Coakley F.V.
      • Kaimal A.
      • Laros Jr, R.K.
      Guidelines for computed tomography and magnetic resonance imaging use during pregnancy and lactation.
      • Kawabata I.
      • Takahashi Y.
      • Iwagaki S.
      • Tamaya T.
      MRI during pregnancy.
      MRI has been used to elucidate fetal anatomy,
      • Estroff J.A.
      The growing role of MR imaging in the fetus.
      • Johnson I.R.
      • Symonds E.M.
      • Kean D.M.
      • et al.
      Imaging the pregnant human uterus with nuclear magnetic resonance.
      • Garden A.S.
      • Griffiths R.D.
      • Weindling A.M.
      • Martin P.A.
      Fast-scan magnetic resonance imaging in fetal visualization.
      • Timor-Tritsch I.E.
      • Monteagudo A.
      Magnetic resonance imaging versus ultrasound for fetal central nervous system abnormalities.
      placental morphology,
      • Warshak C.R.
      • Eskander R.
      • Hull A.D.
      • et al.
      Accuracy of ultrasonography and magnetic resonance imaging in the diagnosis of placenta accreta.
      amniotic fluid volume,
      • Zaretsky M.V.
      • McIntire D.D.
      • Reichel T.F.
      • Twickler D.M.
      Correlation of measured amniotic fluid volume to sonographic and magnetic resonance predictions.
      and biochemical assessment (ie, magnetic resonance spectroscopy).
      • Kok R.D.
      • van den Bergh A.J.
      • Heerschap A.
      • Nijland R.
      • van den Bergh P.P.
      Metabolic information from the human fetal brain obtained with proton magnetic resonance spectroscopy.
      Furthermore, MRI pelvimetry used to be performed to predict consistently women at risk for cephalopelvic disproportion.
      • Fox L.K.
      • Huerta-Enochian G.S.
      • Hamlin J.A.
      • Katz V.L.
      The magnetic resonance imaging-based fetal-pelvic index: a pilot study in the community hospital.
      • Hoyte L.
      • Thomas J.
      • Foster R.T.
      • Shott S.
      • Jakab M.
      • Weidner A.C.
      Racial differences in pelvic morphology among asymptomatic nulliparous women as seen on three-dimensional magnetic resonance images.
      • Huerta-Enochian G.S.
      • Katz V.L.
      • Fox L.K.
      • Hamlin J.A.
      • Kollath J.P.
      Magnetic resonance-based serial pelvimetry: do maternal pelvic dimensions change during pregnancy?.
      • Zaretsky M.V.
      • Alexander J.M.
      • McIntire D.D.
      • Hatab M.R.
      • Twickler D.M.
      • Leveno K.J.
      Magnetic resonance imaging pelvimetry and the prediction of labor dystocia.
      Open-configuration MRI systems were designed to facilitate interventional procedures and functional MRI examinations and to increase patient comfort.
      • Hailey D.
      Open magnetic resonance imaging (MRI) scanners.
      A field strength of 1 Tesla or more is desirable for obtaining high-quality images in open MRI systems.
      Ultrasound is increasingly used to document fetal head position and station within the maternal pelvis at various stages of labor,
      • Bamberg C.
      • Scheuermann S.
      • Slowinski T.
      • et al.
      Relationship between fetal head station established using an open magnetic resonance imaging scanner and the angle of progression determined by transperineal ultrasound.
      • Barbera A.
      • Becker T.
      • Macfarland H.
      • Hobbins J.
      Assessment of fetal head descent in labor with transperineal ultrasound Teaching DVD.
      • Barbera A.F.
      • Pombar X.
      • Perugino G.
      • Lezotte D.C.
      • Hobbins J.C.
      A new method to assess fetal head descent in labor with transperineal ultrasound.
      • Ghi T.
      • Farina A.
      • Pedrazzi A.
      • Rizzo N.
      • Pelusi G.
      • Pilu G.
      Diagnosis of station and rotation of the fetal head in the second stage of labor with intrapartum translabial ultrasound.
      • Henrich W.
      • Dudenhausen J.
      • Fuchs I.
      • Kamena A.
      • Tutschek B.
      Intrapartum translabial ultrasound (ITU): sonographic landmarks and correlation with successful vacuum extraction.
      • Kalache K.D.
      • Duckelmann A.M.
      • Michaelis S.A.
      • Lange J.
      • Cichon G.
      • Dudenhausen J.W.
      Transperineal ultrasound imaging in prolonged second stage of labor with occipitoanterior presenting fetuses: how well does the ‘angle of progression' predict the mode of delivery?.
      • Tutschek B.
      • Braun T.
      • Chantraine F.
      • Henrich W.
      A study of progress of labour using intrapartum translabial ultrasound, assessing head station, direction, and angle of descent.
      but it also has some limitations. We used an open MRI scanner to take images of a human delivery. Our main goal was to describe the relationship between fetal movements and position as the fetus passage through the birth canal, using an open MRI scanner.

      Material and Methods

      We designed the observational study to maximize safety for the mother and fetus. Rupture of the amniotic membranes was not planned in early labor because it was suggested previously that the amniotic fluid could lower the intrauterine acoustic sound pressure by 30 dB.
      • Glover P.
      • Hykin J.
      • Gowland P.
      • Wright J.
      • Johnson I.
      • Mansfield P.
      An assessment of the intrauterine sound intensity level during obstetric echo-planar magnetic resonance imaging.
      This is enough to reduce acoustic sound pressure to an acceptable level (<90 dB). In the late second stage, as the fetal head extended and the perineum distended, cinematic MRI acquisition was terminated to ensure that the ears of the newborn were still covered by maternal soft tissue, thereby avoiding exposure to MRI noise. During the delivery a midwife (G.R.) and an obstetrician (C.B.) stayed in the magnet room (Figure 1) . There were 2 screens inside, 1 to monitor the fetal heart tracing and the second to observe the MRI images. A neonatologist and an anesthetist were also asked to be present in the magnet room. If there had been an abnormal labor course or an emergency, we would have been able to interrupt the MRI birth immediately and transfer to the delivery unit. The MRI suite and the delivery unit are on the same floor, and the distance between them is less than 50 m.
      Figure thumbnail gr1
      FIGURE 1Photograph of the open MRI scanner with the patient and the health care personnel before delivery
      MRI, magnetic resonance imaging.
      Bamberg. Birth in real-time MRI. Am J Obstet Gynecol 2012.
      The patient underwent intermittent electronic fetal heart monitoring with a prototype of a MRI-compatible telemetric system with the exception of the image acquisition time. This MRI-compatible wireless electronic fetal heart rate monitoring prototype system, developed by us and modified from the Philips Avalon Cordless Transducer System (Philips Healthcare, Best, The Netherlands), allows for continuous cardiotocograph tracing with few artifacts. MRI was performed on a 1.0 Tesla open high-field MRI scanner with vertical field orientation (Panorama; Philips Healthcare) using a BodySP-Xl receiver coil.
      A T2-weighted multislice turbospin echo (TSE) single-shot sequence was used to visualize the midsagittal, coronal, and axial planes with the following settings: 1000 milliseconds time of repetition (TR), 100 milliseconds time of echo (TE), flip angle 90°, 40 slices of 6 mm with 1 mm gap, voxel size 1.4 × 1.6 mm, field of view (FOV) 300 × 262 mm, with constant level appearance (CLEAR) correction. The sequences were repeated every 10 minutes during the second stage of labor. Real-time cinematic MRI series were acquired from the midsagittal plane for representation of the extension phase using an interactive TSE single-shot sequence (TR 1600 milliseconds, TE 150 milliseconds, flip angle 90°, single slice of 6 mm, voxel size 1.4 × 1.5 mm, FOV 380 × 285 mm, with CLEAR correction).
      A final MRI sequence was performed immediately after childbirth using a balanced fast field echo sequence (TR 6 milliseconds, TE 3 milliseconds, flip angle 60°, 26 slices of 6 mm with no gap, voxel size 2.0 × 2.03 mm, FOV 340 × 340 mm) to evaluate the third stage of labor with regard to placental separation and uterus involution.
      This study design was ethically approved and written informed consent to participate was obtained from the patient at 28 weeks of gestation. In the counseling process, the patient was informed that MRI appears to be safe for the mother and the fetus because no reports of adverse effects have been made.

      Results

      In November 2010, a 24 year old gravida 2, para 2 woman at 37 5/7 weeks of gestation was admitted with regular contractions to the Department of Obstetrics of the Charité University Hospital in Berlin, Germany. The patient received an epidural and was transferred to the open MRI suite. In addition, the cervix was fully dilated, and the presenting part was engaged. Eight MRI studies were performed over a period of 45 minutes: 7 antepartum studies (Figure 2) and 1 postpartum study. First, the woman was examined in the supine position with legs outstretched. In the active second stage, when the mother began expulsive efforts with the valsalva manoeuver, her legs were slightly abducted and supported by padding. This period was evaluated by real-time cinematic MRI series (Video Clip).
      Figure thumbnail gr2
      FIGURE 2View of the midsagittal MRI plane of the maternal pelvis before the expulsion phase without pushing
      The fetal head station is midpelvis (in comparison with standard obstetric textbooks

      Sellheim H. Die Geburt des Menschen nach anatomischen, vergleichend-anatomischen, physiologischen, physikalischen, entwicklungsmechanischen, biologischen und sozialen Gesichtspunkten. Vol 1. Gießen (Germany): Deutsche Frauenheilkunde, E. Opitz; 1913.

      • Dudenhausen J.W.
      Praktische geburtshilfe.
      • Hanretty K.P.
      Obstetrics illustrated.
      ) and the membranes are intact.
      MRI, magnetic resonance imaging.
      Bamberg. Birth in real-time MRI. Am J Obstet Gynecol 2012.
      A 2585 gram appropriate-for-gestational age boy with Apgar scores of 9, 9, and 10 at 1, 5, and 10 minutes. Umbilical artery and umbilical vein pH measurements are routinely assessed as part of our daily practice. However, because of technical difficulties with the umbilical artery blood sample in this case, only the umbilical vein pH was available, which was 7.32. A neonatologist assessed the condition of the baby. Immediately after childbirth, the maternal anatomy was imaged before and after expulsion of the placenta, using a BFFE sequence (Figure 3) . The total individual study time in the magnet room was less than 1 hour. The woman tolerated the discomfort during labor well and her postpartum course was uneventful. She was discharged with her newborn 2 days after delivery. The pediatric screening examinations, including auditory tests, did not reveal any abnormalities.
      Figure thumbnail gr3
      FIGURE 3MRI examination of the maternal pelvis in the third stage of labor
      Left panel shows the separated placenta located in the lower uterine segment and vagina just before expulsion. Right panel shows uterus in the midsagittal view directly after the delivery of the placenta with an empty uterine cavity.
      MRI, magnetic resonance imaging.
      Bamberg. Birth in real-time MRI. Am J Obstet Gynecol 2012.

      Comment

      The mechanical factors that influence the progress of labor are of interest to obstetricians, but they are often difficult to investigate. For many years, digital examination was the only method that was used during labor to provide information about the mother's bony pelvis and soft tissue and the fetus. This method has the disadvantage that only limited areas of the fetus and birth canal can be assessed.
      Because mechanical factors are primarily involved in the seven cardinal movements of labor
      • Norwitz E.R.
      • Robinson J.N.
      • Repke J.T.
      Labor and delivery.
      • Liao J.B.
      • Buhimschi C.S.
      • Norwitz E.R.
      Normal labor: mechanism and duration.
      (engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion) elucidation of the process of labor was also investigated by experimental studies on preserved pelvises and pelvic models. Anatomically correct models are imperative for accurate simulations of normal and complicated deliveries.
      Clearly, it is impossible to faithfully reproduce labor conditions in experiments involving models; therefore, conclusions based on the results of such studies remain hypothetical.
      • Dietze M.
      A re-evaluation of the mechanism of labour for the contemporary midwifery practice.
      In other words, it is difficult to generate models that mirror accurately the in vivo relationships during the labor and delivery process, and hence, results derived from simulation are often based on untested assumptions.
      Ultrasound is the imaging modality of choice for pregnant women.
      • Levine D.
      Obstetric MRI.
      Today the cardinal movements can be studied with sonography.
      • Akmal S.
      • Tsoi E.
      • Howard R.
      • Osei E.
      • Nicolaides K.H.
      Investigation of occiput posterior delivery by intrapartum sonography.
      • Akmal S.
      • Tsoi E.
      • Kametas N.
      • Howard R.
      • Nicolaides K.H.
      Intrapartum sonography to determine fetal head position.
      • Gardberg M.
      • Laakkonen E.
      • Salevaara M.
      Intrapartum sonography and persistent occiput posterior position: a study of 408 deliveries.
      • Souka A.P.
      • Haritos T.
      • Basayiannis K.
      • Noikokyri N.
      • Antsaklis A.
      Intrapartum ultrasound for the examination of the fetal head position in normal and obstructed labor.
      Transperineal ultrasound is rapidly becoming an established method to assess progression of labor and the likelihood of a successful operative vaginal delivery.
      • Yeo L.
      • Romero R.
      Sonographic evaluation in the second stage of labor to improve the assessment of labor progress and its outcome.
      However, specific bony landmarks of the maternal pelvis, such as the ischial spines, cannot be visualized by intrapartum ultrasound.
      • Bamberg C.
      • Scheuermann S.
      • Slowinski T.
      • et al.
      Relationship between fetal head station established using an open magnetic resonance imaging scanner and the angle of progression determined by transperineal ultrasound.
      Furthermore, it is impossible to evaluate the fetal attitude which is described as the degree of flexion or extension of the fetal head in relation to the fetal spine
      • Liao J.B.
      • Buhimschi C.S.
      • Norwitz E.R.
      Normal labor: mechanism and duration.
      because the fetal cervical spine is not visible by transperineal ultrasound.
      Fetal MRI has become more widely available and has become an accepted and powerful complementary method for evaluation of the fetus.
      • Estroff J.A.
      The growing role of MR imaging in the fetus.
      MRI can help to increase knowledge about maternal and fetal anatomy during labor. Bony structures as well as soft tissue could be assessed in great detail. MRI sequences of the birth process should be as quite as possible and resistant to movement artefacts. Our preliminary experiments confirmed the single-shot TSE sequence to be well suited for fetal imaging for these tasks; sequence optimization also aimed at minimizing the sound pressure level, which was given by the scanner in the range of 10.5–11.3 dB (Table) .
      TABLETechnical MRI definitions
      • Westbrook C.
      Handbook of MRI technique.
      • Westbrook C.
      • Kaut C.
      MRI in practice.
      TeslaField strength of the magnets used for MRI is measured in units of Tesla
      SequenceAn ordered combination of radio frequencies and gradient pulses designed to aquire the data to form the image
      Time of echo (TE)The time between a radio frequency pulse and the collection of the signal
      Time of repetition (TR)The time from the application of 1 radio frequency pulse to the application of the next
      Turbo spin echo (TSE) single-shot sequenceSpin echo pulse sequence with scan times that are drastically shorter than conventional spin echo
      T2-weighted imageA standard basic MRI, in which water appears brighter and bones darker. It requires long TE and long TR.
      MRI, magnetic resonance imaging.
      Bamberg. Birth in real-time MRI. Am J Obstet Gynecol 2012.

      Attitude-fetal flexion or extension

      In 1913, the German obstetrician Hugo Sellheim

      Sellheim H. Die Geburt des Menschen nach anatomischen, vergleichend-anatomischen, physiologischen, physikalischen, entwicklungsmechanischen, biologischen und sozialen Gesichtspunkten. Vol 1. Gießen (Germany): Deutsche Frauenheilkunde, E. Opitz; 1913.

      found that extension of the fetal head takes place after impingement of the suboccipital region of the fetal head on the mother's symphysis. This was thought to be caused by rotation around a transverse axis running through the lower border of the symphysis, resulting in deflexion of the fetal head at the atlantooccipital joint and extension of the cervical spine. This movement begins before crowning. In 1957, the radiological investigations of Borell and Fernström
      • Borell U.
      • Fernström I.
      The movements in the mechanism of disengagement with special reference to the attitude of the foetal head.
      • Borell U.
      • Fernström I.
      Shape and course of the birth canal; a radiographic study in the human.
      suggested that Sellheim's explanation of the extension was incorrect. Borell and Fernström suggested that the fetal head is flexed and glides continuously downward. The thoracic and cervical spine undergoes extension during the last stage of labor, whereas the fetal head remains flexed until after expulsion. Furthermore, they stated that there is a relatively large distance between the head and the lower border of the symphysis.
      An intermediate position between these 2 theories was developed by Murray (1890)
      • Murray R.M.
      The axis-traction forceps: The mechanical principles, construction, and scope.
      and Jones (1906).
      • Jones J.
      Some causes of delay in labour; with special reference to the function of the cervical spine of the fetus.
      The view of Murray
      • Murray R.M.
      The axis-traction forceps: The mechanical principles, construction, and scope.
      was that the head continues to glide downward at the same time as the movement of extension. According to Jones,
      • Jones J.
      Some causes of delay in labour; with special reference to the function of the cervical spine of the fetus.
      the movement of extension does not occur merely at the articulation between the occiput and the atlas but is preceded by an extension of the entire cervical spine.
      Our visualization of the normal mechanism of late second-stage labor by MRI shows that extension started as soon as the occiput was in close contact with the inferior margin of the symphysis pubis. Thereafter, extension was simultaneous with gliding downward of the fetal head. At this point, the birth canal curved 90° upward and the fetal head was delivered by extension and rotated around the symphysis pubis. To the best of our knowledge, this is the first time that this mechanism has been clearly visualized. Thus, our investigation of the mechanism of labor using real time MRI produced results that are in line with Murray's and Jones's theory, which were described more than a century ago.
      • Murray R.M.
      The axis-traction forceps: The mechanical principles, construction, and scope.
      • Jones J.
      Some causes of delay in labour; with special reference to the function of the cervical spine of the fetus.

      MRI-compatible cardiotocography

      Poutamo et al
      • Poutamo J.
      • Partanen K.
      • Vanninen R.
      • Vainio P.
      • Kirkinen P.
      MRI does not change fetal cardiotocographic parameters.
      published a comparison of electronic fetal heart monitoring before and after MR imaging in 16 pregnant women. The authors showed that MRI acquisition does not influence the fetal heart rate or fetal activity. Shakespeare et al
      • Shakespeare S.A.
      • Moore R.J.
      • Crowe J.A.
      • Gowland P.A.
      • Hayes-Gill B.R.
      A method for foetal heart rate monitoring during magnetic resonance imaging using Doppler ultrasound.
      and Vadeyar et al
      • Vadeyar S.H.
      • Moore R.J.
      • Strachan B.K.
      • et al.
      Effect of fetal magnetic resonance imaging on fetal heart rate patterns.
      were the first to assess fetal well-being during MRI examination. They had to remove ferromagnetic parts from the device to be able to use within the MRI scanner. These authors also found no visible effect of MRI on fetal heart rate patterns. In the present observation, the same approach regarding the cardiotocography was chosen, namely removing ferromagnetic parts to allow monitoring of fetal heart rate during MRI examination without interference.

      Future research

      Future research should include visualization of the first stage of labor by MRI. Arrest of labor, necessitating a cesarean delivery, is a major cause of maternal morbidity and mortality.
      • Cheng Y.W.
      • Shaffer B.L.
      • Bryant A.S.
      • Caughey A.B.
      Length of the first stage of labor and associated perinatal outcomes in nulliparous women.
      • Cheng Y.W.
      • Hopkins L.M.
      • Laros Jr, R.K.
      • Caughey A.B.
      Duration of the second stage of labor in multiparous women: maternal and neonatal outcomes.
      An improved understanding of the mechanism of labor will help clinicians towards a more individualized approach to labor, allowing them to more easily distinguish normal and abnormal courses of labor. This knowledge would allow clinicians to intervene in a timely and effective fashion to ensure a favorable outcome. Furthermore, a basic knowledge of the attitude of the fetal head at the time of its passage through the lower part of the birth canal is of practical value in operative vaginal deliveries. Future studies might also provide a basis for virtual reality computer programs to teach health care personnel in training.
      The shape and direction of the birth canal has generally been investigated by palpation during labor and in frozen sections from women who died during labor. There is no doubt that the human fetuses must negotiate a curve to be born.
      • Norwitz E.R.
      • Robinson J.N.
      • Repke J.T.
      Labor and delivery.
      Nevertheless, there are conflicting results about the level of the curved part of the birth canal, the so-called knee. It is widely reported in textbooks, but not supported by evidence, that the knee lies at the level of the ischial spines.
      • Oxorn H.
      • Foote W.R.
      In contrast, Borell and Fernström
      • Borell U.
      • Fernström I.
      The mechanism of labour.
      stated that the curve of the birth canal lies lower and outside the bony pelvis and is entirely formed by the soft parts. MRI visualization of the ischial spine level during labor in comparison with the fetal head station during the extension phase may help to shed more light on this discussion.
      In conclusion, collecting images of the fetus during delivery using an open MRI is feasible. We showed that MRI technology is useful for visualizing normal maternal and fetal anatomy during labor. This observation opened a new way to study the mechanism of birth.

      Supplementary data

      • Video Clip

        Accelerated real-time cinematic MRI series

        Accelerated real-time cinematic MRI series in an open high-field scanner during the active second stage of labor, when the mother starts performing expulsive efforts with the valsalva maneuver. The midsagittal plane is shown and the amniotic membranes are intact. In the late second stage, as the fetal head extended and the perineum stretched, cinematic MRI acquisition was terminated to ensure that the ears of the newborn were still covered by maternal soft tissue, thereby avoiding exposure to MRI noise.

        MRI, magnetic resonance imaging.

        Bamberg. Birth in real-time MRI. Am J Obstet Gynecol 2012.

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