Ultrasound in Obstetrics

Chapter 44. Ultrasound in Obstetrics

Oct. 1996. 1st year resident J. W. Lee. M.D.


I. Ultrasonography


A. Clinical Application


  1. Very early identification of IUP.
  2. Demonstration of the size & the rate of growth of the amnionic sac and embryo.
  3. Identification of multiple pregnancy.
  4. Measurement of fetal head, AC, FL, & other anatomical landmark to help identify the duration of gestation.
  5. Comparison of fetal head & chest / abd. circumference --> identify hydrocephaly, microcephaly, or anencephaly.
  6. Detection of fetal anomaly.
  7. Demonstration of AFV.
  8. Identification of location of size of placenta.
  9. Demonstration of placental abnormalities.
  10. Identification of uterine tumor / anomaly.
  11. Detection of FB .. IUD, blood clots, retained placenta.


B. Guidelines for Real-time Sonography



  • Table 44-1

  • The American Institute of Ultrasound in Medicine (1991)

    : specific recommendations for the content of fetal anatomical surveys

    --> cerebral ventricles, 4 chamber view of the heart, spine, stomach, UB, umbilical cord insertion on the abd. wall, & renal regions.


C. Selected Vs Routine USG



  • Advantages of routine screening

    : less frequent labor induction for postterm pregnancy, detection of IUGR & fetal anomaly.



  • NIH (1984) : USG during pregnancy

    --> safe, but should be performed only when specially indicated.


D. Fetal & Amnionic Fluid measurement



  • The most commonly measured dimensions :CRL, BPD, AC, FL.

    cf. HC, length of humerus, ulna, tibia, clavicle, binocular distance.



  • Most prediction of G-age : based upon the 50th percentile measurement observed in
    normal fetuses.



  • AFI : values in percentiles for pregnancies between 16 & 42weeks.


E. Fetal Anatomy



  • Most attempts to survey fetal anatomy using USG : during the early 2nd trimester or
    later.
  • Detection of fetal anomalies during 1st trimester

    (Caution)


    1. Normal 1st-trimester embryological development may mimic pathological changes in the
      2nd & 3rd trimester. (ex. physiological ant. wall herniation vs omphalocele)
    2. Grossly abnormal embryos may appear normal. (ex. anencephaly)
    3. Some abnormal embryos may manifest only with CRL that is less than expected for their G-age.


F. Cardiac Anomalies




  • Incidence : 8/1.000 livebirths, 27 ~ 77/1.000 stillborns.

  • Recommendation of detail fetal heart exam.
    : Nonimmune hydrops, suspected abnormality seen by screening USG, teratogen exposure, parenteral / sibling heart defect, aneuploidy, extracardiac anomaly, maternal DM, fetal arrhythmia.

  • Frequently a/w aneuploidy.

  • Heart motion can be visualized sonographically in a 6- to 7-week embryo, but detail cardiac exam. is usually not possible until at least 18wk.

  • 4-chamber view : central to fetal cardiac assessment.

    --> 96% with sonographically detectable heart defects : had abnormal 4-chamber view.

  • M-mode echocardiography : to measure chamber size, wall thickness, and wall & valve motion, and to facilitate assessment of cardiac arrhythmias.

  • Heart Dz. diagnosed prenatally is usually severe and is a/w a poor long-term prognosis.


G. CNS Abnormalities




  • 3 Routine transverse / axial sonographic views recommended by Nyberg (1989) to depict the fetal brain & cranium.


  1. Transthalamic view : used to measure BPD, HC, includes the thalamus, cavum septum pellicidum, & the frontal horn of lat. ventricle.
  2. Transventricular view : contain lat. ventricle, choroid plexus.
  3. Transcerebellar view : obtained by angling through the post. fossa in the suboccipital bregmatic plane and demonstrates the cerebellum and cisterna magna.


1. Hydrocephalus



  • Incidence : 03 ~ 0.8 / 1000 births
  • USG :


  1. identify abnormally large head


  2. by determining the lat. ventricular ratio

    cf.. In normal fetus, lat. ventricular atrium dia. : relatively constant from 6 to 9 mm btw 18 & 35wk. but if atrial dia. > 10mm --> suggest ventriculomegaly.


    cf.. Dilatation of the ventricular atrium & occipital horns

    --> thought to be an early event in the development of fetal hydrocephalus.


  3. Relationship btw the choroid plexus & lat. wall of the dependent lat. ventricle.



2. Neural-tube Defects



  • Result from failure of tube closure by the 6th G-week (embryonic age 26 to 28 days)


  • Anencephaly



    • Dx : theoretically possible as early as 8 weeks.
    • Absence of the cranial vault and brain above the base of the skull & orbits.
    • Hydramnios, secondary to impaired fetal swallowing, but late finding.



  • Encephalocele : condition in which the meninges & CSF, usually in a/w brain tissue, herniate through a cranial defect.




    • most commonly results from an occipital midline defect.
    • a/w either hydrocephaly / microcephaly.
    • important feature of Meckel-Gruber syndrome and frequently accompanies amnionic band syndrome



  • Spina bifida : consist of a hiatus, usually in the lumbosacral vertebrae, through which a meningeal sac may protrude, forming a meningocele.


    . Fetal spine should be examined by USG with sagittal, transverse, & coronal views.

    . a/w cranial abnormalities.

    ex.) Arnold-Chiari malformation


    : the medulla & 4th ventricle are elongated and , in combination with a portion of the cerebellum, extend through the foramen magnum into the upper cervical spinal canal.


    : USG --> Ventriculomegaly, frontal bone scalloping (lemon sign), abnormal
    curvature of the cerebellum (banana sign), decreased cerebellar size, and failure to visualize the cerebellum and obliteration of cisterna magna.



  • Choroid Plexus Cysts : usually, transient and of no clinical significance


    cf. in some situation, a/w trisomy 18.


H. GI Abnormalities


  • Using HR-USG, the integrity of the abd. wall, umbilical cord insertion, & intraabdominal anomalies can be assessed with confidence.

  • Liver, spleen, GB, & bowel can be identified in most pregnancies.


1. Diaphragmatic Hernia.




  • results from incomplete fusion of the pleuroperitoneal membrane, occurs more frequently on the left.
  • Incidence : 1/ 2000 ~ 3000 births
  • USG

    1. Cystic structure, usually the left atrium is seen on an axial image at the level of the 4-chamber cardiac view.
    2. Absence of an intra-abd. stomach bubble
    3. Mediastinal shift usually with a normal cardiac axis
    4. Small abd. circumference
    5. Peristalsis in the fetal chest -- the most specific findings.
    6. Associated hydramnios : 75%.
    7. Cause of neonatal death : respiratory failure secondary to pulmonary hypoplasia or from associated lethal anomalies.


2. Abd. Wall Defect.




  • Incidence : 1/5000 livebirths


# Omphalocele




  • Congenital midline defect in the ant. abd. wall that result when the lat. abd. folds fail to fuse.
  • Diagnosed accurately as early as 12 weeks
  • Results in herniation of intra-abd. structures into the base of the umbilical cord.
  • Dx. by USG : a sac containing intraabdominal structures is imaged outside the abdomen.
  • a/w anomalies : 50%
  • Px. : depend on largely on other major malformations or chromosomal abnormalities.


# Gastroschisis




  • Intra-abd. organs herniate through a defect in the ant. abd. wall.


  • Caused by interruption of the right omphalomesenteric artery and is located usually to the right of the umbilicus and spares the rectus muscle.


  • USG - extracorporeal bowel : typical cauliflower appearance.

    - herniated organs : float freely in the AF with no covering membrane, and the umbilical cord insertion is normal.


  • a/w anomalies (20 ~ 40 %)


3. GI Atresia.




  • In general, the more proximal the obstruction, : a/w hydramnios.

    ex.. Esophageal atresia & TEF : cannot be diagnosed reliably in utero.

    Duodenal atresia : diagnosed perinatally by the demonstration of the double-bubble sign. (distention of the stomach & the 1st part of the duodenum)


  • Obstructions in the lower small bowel : multiple dilated loops & increased peristaltic activity.


  • Large bowel obstruction & anal atresia : less readily diagnosed in utero because hydramnios is not a typical feature and the bowel may not be significantly dilated.



I. GU Tract.




  • Fetal kidneys : visualized as paraspinous masses frequently as early as 14 weeks and routinely by 18 weeks.
  • Fetal bladder : detectable 2nd trimester. (d/t urine production normally begin late in the 1st trimester)
  • In normal fetus, bladder fills and empties every 20 to 45 minutes.
  • Normal AFV : suggest urinary tract patency with at least one functioning kidney, esp. in the 2nd half of pregnancy.
  • Before 16 ~ 20 weeks, there may be normal AFV despite absent renal function, although otherwise unexplained oligohydramnios still suggests a urinary abnormality.


  • Table 44-7.


1) Obstructive Uropathy.




  • Fetal urinary tract obstruction : develop at the UPJ, UVJ, or at the urethra.
  • The severity & location of the obstruction can be determined by assessing AFV as well as the distribution of any resulting urinary tract dilatation.
  • As a general rule, fetal ureters are not visualized, thus their identification indicates hydroureter.


# Renal Agenesis



  • Bilat. renal agenesis : 1/4000 births
  • a/w severe oligohydramnios
  • Characteristics : pul. hypoplasia, limb deformities, loose skin, typical faces of the Potter syndrome.


# Cystic Renal Dz.




  • Depending on the timing and severity of urinary obstruction secondary cystic changes may develop in th fetal kidneys.
  • Potter type Ⅱ : classic multicystic dysplastic kidney.



II. Doppler Velocimetry



  • Doppler shift principle : echo returning from moving structures are altered in frequency and the amount of shift is directly proportional to the velocity of the moving structure.


  • 2 Methods of estimating circulatory hemodynamics




    1. Direct measurement of the volume of blood flow.
    2. Indirect estimation of flow velocity using wave form analysis.


A. Determination of Blood Volume Flow.




  • Figure 44-13


    -- c : the speed of sound (in tissue. equal to 1540 m/sec)





B. Waveform Analysis of Blood Velocity




  • Figure 44-14



  • Maternal uterine artery / fetal umbilical artery : S/D ratio gradually decrease as gestation progresses.

    cf.. more central fetal vessels, such as the descending aorta, --> because of the low diastolic velocities seen, S-D ratio is not useful elsewhere in the fetal circulation.



  • Pourcelot index / Resistance index : applicable only to the umbilical and uterine artery, and low diastolic values limit its usefulness in the fetal aorta or other central vessels.



  • Pulsatility index.




    • require a digitized waveform for calculating the mean of the maximal frequencies represented.


    • Because of the mean value in the denominator, this index can be computed using flow data from the fetal descending aorta without encountering excessive variation caused with division by small numbers as with the other two indecies.



  • During pregnancy, maternal & fetal vessels perfusing the placenta assume waveforms indicative of continuous diastolic flow.



  • Waveforms with a high flow in diastole accompany low downstream vessel impedance. In contrast, waveforms with little diastolic flow, or reversed flow, are seen when vascular impedance downstream is abnormally high (e.g. placental insufficiency).



  • Figure 44-15



C. Techniques for Specific Vessels



1. Uterine & Arcuate Arteries.




  • Placental circulation : high volume flow, with an extensive diastolic component.
  • Uterine blood flow : 50mL/min shortly after conception, --> 500 ~ 750 mL/min by term.
  • High diastolic velocities & highly turbulent flow
  • With this degree of diastolic flow, indices decrease as term approaches - that is, diastolic velocity increase with advancing gestation.


2. Umbilical Vein




  • Intra-abd. portion of the umbilical vein : relatively straight & flow tends to be constant rather than pulsating.
  • A reasonable vessel for measuring volume flow rather than indexed flow.
  • Umbilical vein blood-flow measurements range from 108 to 153 m/kg / min. with
    decreasing flows as gestation advances.


3. Fetal Descending Aorta.




  • Highly pulsatile, with little diastolic flow.
  • Waveform flow measurement : limited to the PI because of the lack of diastolic flow.
  • Blood flow range : 185 ~ 246mL/kg/min.


4. Carotid Artery & CBF



  • Carotid arteries : the only cerebral vessels that can be identified reliably, but they are too narrow to measure accurately.




III. Potential Clinical Applications of Doppler Velocimetry


A. Fetal Cardiac Function



  1. Cardiac Output



    • extremely difficult to apply because of fetal movement and technical factors


  2. Ductus Arteriosus.


  3. Fetal Arrhythmias


    • By 2-D real-time US + M-mode ECG.


B. Fetal Well-being



  • Predictions of growth retardation, fetal hypoxia, and fetal distress.
  • Abnormal uteroplacental waveforms may identify women at risk for PIH.


1. Fetal Growth Retardation.


  • Fetal aortic / umbilical artery circulation as well as uteroplacental waveforms have been studied in attempts to predict fetal growth retardation.


2. Fetal Distress

  • A test to predict fetal asphyxia before or during labor would be of great value and attempts have been made to apply Doppler velocimetry in this capacity.


  • Unfortunately, velocimetry has not yet been proved to be a good predictor of antepartum / intrapartum fetal condition because of wide random variations in Doppler results.


3. Prediction of Hypertension.



  • Abnormal uteroplacental waveforms may reflect the raised vascular resistance due to maternal hypertension in the spiral arterioles that support the placental circulation.


C. Summary


  • At present, the clinical utility of Doppler velocimetry has not been proven conclusively.