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                                              Obstetrical Ultrasound


What are the Types of Obstetrical Ultrasound Imaging?


First Trimester


Indications for a First trimester ultrasound: First Trimester ultrasound can be used: 

  • To confirm the presence of an intrauterine pregnancy.

  • To evaluate a suspected ectopic pregnancy. To define the cause of vaginal bleeding.

  • To evaluate pelvic pain.

  • To estimate gestational (menstrual) age when last menstrual period date is uncertainTo diagnose or evaluate multiple gestations and reliable determination of chorionicity or amnionicity.   

  • To confirm cardiac activity. As an adjunct to chorionic villus sampling, embryo transfer, localization and removal of an intrauterine device, and prior to cervical cerclage placement.

  • To assess for certain fetal anomalies, such as anenecephaly, in high-risk patients. To evaluate maternal pelvic masses and/or uterine abnormalities.

  • To measure nuchal translucency (NT) when part of a screening program for fetal aneuploidy.

  • To evaluate a suspected hydatidiform mole.

  • To evaluate a threatened abortion to document fetal viability or for incomplete abortion to identify retained products of conception.         


Scanning in the first trimester may be performed either transabdominally or transvaginally. If a transabdominal examination is not definitive, a transvaginal scan or transperineal scan should be performed whenever possible.


  • The uterus, including the cervix, and adnexa should be evaluated for the presence of a gestational sac. If a gestational sac is seen, its location should be documented. The gestational sac should be evaluated for the presence or absence of a yolk sac or embryo, and the crown-rump length should be recorded, when possible. First-trimester crown-rump measurement is the most accurate means for sonographic dating of pregnancy.

  • Presence or absence of cardiac activity

  • Fetal number; Amnionicity and chorionicity should be documented for all multiple gestations

  • Embryonic/fetal anatomy appropriate for the first trimester

  • The uterus, including the cervix, adnexal structures, and cul-de-sac


First Trimester Screening


For those patients desiring to assess their individual risk of fetal aneuploidy, a very specific measurement of the NT during a specific age interval is necessary. Ultrasound at 10 to 13 6/7 weeks or with crown–rump length from 45 mm to 84 mm, can quantify the risk of Down syndrome and other genetic abnormalities using nuchal translucency (NT) measurement. The term NT describes a sonolucent area in the posterior fetal neck (fluid beneath the skin behind baby’s neck).


Increased NT is associated with trisomy 21, 18, 13, and certain other chromosomal or developmental abnormalities, and numerous genetic syndromes. In particular, for chromosomally normal fetuses with increased nuchal translucency, there is a higher risk of congenital heart disease, and a properly timed and careful review of fetal heart anatomy is recommended.


Nuchal translucency screening should only be offered as part of a comprehensive prenatal screening and counseling program by experienced operators with appropriate quality assurance processes in place. This non-invasive procedure combines the results from the blood tests and the ultrasound, along with the mother’s age, to determine risk factors.


Limited Ultrasound


A limited examination is performed when a specific question requires investigation. A limited examination may be performed in clinical emergencies or for a limited purpose such as evaluation of fetal or embryonic cardiac activity, fetal position, or amniotic fluid volume. For example, a limited examination could be performed to confirm fetal heart activity in a bleeding patient or to verify fetal presentation in a laboring patient. 


Indications for a Limited OB Ultrasound:

  • no fetal movement or decreased fetal movement > 24 weeks gestation

  • vaginal bleeding 

  • verifying fetal presentation in patient who is in labor outside of the hospital or >35 weeks gestation

  • pelvic pain in pregnancy

  • assessment of amniotic fluid volume in cases of oligohydramnios  (AFI < 5cm or the absence of a fluid pocket 2 cm in depth) and polyhydramnios (AFI) > 24 cm or a single pocket of fluid at least 8 cm in depth

  • placental localization in cases of suspected previa

  • evaluation of certain placental abnormalities  (abruption)

  • follow-up of growth of uterine fibroids (submucosal/intramural interfering with intrauterine growth)

  • patients with uncertain dates


A limited ultrasound may include:

  • Presence or absence of cardiac activity

  • Fetal anatomy

  • Assessment of the amniotic fluid

  • Fetal position

  • Assessment of the placenta

  • Assessment of the uterus, including the cervix, or adnexal structures        


Standard Obstetric Ultrasound


Traditional ultrasound exam uses a transducer over the abdomen to generate 2-D images of the developing fetus.


  1. A standard obstetric ultrasound includes:

  2. Fetal cardiac activity, fetal number, and presentation.

  3. A qualitative or semiqualitative estimate of amniotic fluid volume.

  4. The placental location, appearance, and relationship to the internal cervical os

  5. The umbilical cord should be imaged, and the number of vessels in the cord should be evaluated

  6. Gestational (menstrual) age assessment. Beyond the first trimester, a variety of sonographic parameters such as biparietal diameter, head circumference, abdominal circumference, and femoral diaphysis length can be used to estimate gestational (menstrual) age. The variability of gestational (menstrual) age estimations, however, increases with advancing pregnancy. Significant discrepancies between gestational (menstrual) age and fetal measurements may suggest the possibility of a fetal growth abnormality, intrauterine growth restriction, or macrosomia.

  7. Fetal weight estimation. Fetal weight can be estimated by obtaining measurements such as the biparietal diameter, head circumference, abdominal circumference or average abdominal diameter, and femoral diaphysis length. Results from various prediction models can be compared to fetal weight percentiles from published nomograms. Currently, even the best fetal weight prediction methods can yield errors as high as ±15%.

  8. Maternal anatomy. Evaluation of the uterus, adnexal structures, and cervix should be performed when appropriate. When the cervix cannot be visualized, a transperineal or transvaginal scan may be considered when evaluation of the cervix is needed.

  9. Fetal anatomic survey. The following areas of assessment represent the minimal elements of a standard examination of fetal anatomy. A more detailed fetal anatomic examination may be necessary if an abnormality or suspected abnormality is found on the standard examination.         

i. Head, face, and neck

Cerebellum, Choroid plexus, Cisterna magna, Lateral cerebral ventricles, Midline falx, Cavum septi pellucidi, Upper lip          

ii. Chest         

The basic cardiac examination includes a 4-chamber view of the fetal heart.          

iii. Abdomen

Stomach (presence, size, and situs), Kidneys, Bladder, Umbilical cord insertion site into the fetal abdomen with umbilical cord vessel number          

iv. Spine         

Cervical, thoracic, lumbar, and sacral spine          

v. Extremities            

Legs and arms: presence or absence


(Comprehensive) Level II Obstetric Ultrasound 


This exam is similar to the standard ultrasound, but the exam targets a suspected problem and uses more sophisticated equipment. This ultrasound is performed for a known or suspected fetal anatomic, genetic abnormality (i.e.,previous 

anomalous fetus, abnormal scan this pregnancy, etc.) or increased risk for fetal abnormality (eg. AMA, diabetic, fetus at risk due to teratogen or genetics, or abnormal prenatal screen).


The comprehensive obstetric ultrasound includes all of the components of the standard ultrasound, plus a detailed fetal anatomical survey.


Evaluation of intracranial, facial and spinal anatomy: 

  • Lateral ventricles, third and fourth ventricles 

  • Cerebellum, integrity of lobes, vermis 

  • Cavum septum pellucidum 

  • Cisterna magna measurement

  • Nuchal thickness measurement (15‐20  weeks)  

  • Integrity of cranial vault 

  • Examination of brain parenchyma, (e.g. for calcifications) 

  • Ear position, size 

  • Face  

  • Upper lip integrity 

  • Palate

  • Facial profile

  • Evaluation of the neck (e.g. for masses)  


Evaluation of the chest:  

  • Presence of masses 

  • Pleural effusion 

  • Integrity of both sides of the diaphragm

  • Appearance of ribs 


Evaluation of the heart:  

  • Cardiac location and axis 

  • Outflow tracts 


Evaluation of the abdomen:

  • Bowel 

  • Adrenal glands 

  • Gallbladder 

  • Liver 

  • Spleen 

  • Ascites 

  • Masses 


Evaluation of genitalia:  

  • Gender (whether or not parents wish to know sex of child) 


Evaluation of limbs:  

  • Number, size, and architecture

  • Anatomy and position of hands 

  • Anatomy and position of feet 


Evaluation of the placenta and cord:  

  • Placental cord insertion site 

  • Placental masses 

  • Umbilical‐cord (number of arteries) 


Evaluation of amniotic fluid:  

  • Amniotic Fluid Index

  • Evaluation of the cervix (Not required) 

  • Evaluation of the maternal adnexa when feasible


Follow-up Ultrasound


This includes performing a focused assessment of fetal size by measuring the BPD, abdominal circumference, femur length, or other appropriate measurements, OR a detailed re‐examination of a specific organ or system known or suspected to be abnormal.  


Indications for a follow-up OB ultrasound:

  1. serial growth assessment in cases of documented IUGR (frequency no less than every 2 weeks)

  2. size/dates discrepancy (small for gestational age fetus, large for gestational age fetus)

  3. follow-up of detected fetal structural abnormalities (rule out fetal hydrops)

  4. follow-up by a MFM of poorly visualized fetal anatomic structures from a previous standard or targeted ultrasound examination

  5. multiple gestation

  6. maternal medical condition associated with risk of poor fetal growth with size dates discordance (hypertension, chronic renal disease, connective tissue disorder, diabetes mellitus ( uncontrolled pregestational or gestational), antiphospholipid antibody syndrome, inflammatory bowel disease, sever malnutrition, hyperthyroidism)


Other Ultrasound Examinations:


Biophysical Profile (BPP)

A biophysical profile (BPP) test measures the health of your baby (fetus) during pregnancy. A biophysical profile uses ultrasound to check on a baby's well-being. A BPP test may include a nonstress test with electronic fetal heart monitoring and a fetal ultrasound. The BPP measures your baby's heart rate, muscle tone, movement, breathing, and the amount of amniotic fluid around your baby. These are evaluated and given a score. A low score on a biophysical profile might indicate that you and your baby need further monitoring or special care. In some cases, early or immediate delivery might be recommended. A high score on a biophysical profile is reassuring and continued surveillance of your pregnancy is recommended.


Doppler Ultrasound

This imaging procedure measures slight changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells. Several types of


Doppler ultrasound of fetal blood vessels can be performed:

  • Doppler ultrasound of the fetal umbilical arteries (umbilical cord) can be performed when there is fetal growth restriction present or placental dysfunction.

  • Doppler ultrasound of the fetal middle cerebral artery (fetal head) can be performed when there is fetal growth restriction or fetal anemia suspected.

  • Doppler ultrasound of the ductus venosus (fetal abdomen) can be performed when there is fetal growth restriction or cardiac dysfunction suspected.


Transvaginal Ultrasound


Specially designed probe transducers are used inside the vagina to generate sonogram images.

  • Transvaginal Ultrasound can be used to assess the early stages of pregnancy.

  • Transvaginal ultrasound can also be used to measure the cervical length when patients have a history of preterm delivery or when there is suspicion of a short cervix.

  • Transvaginal ultrasound may also be performed when the patient has bleeding or the placenta is close to or covering the cervix (placenta previa)


3D/4D Ultrasound


Standard ultrasound pictures are taken in 2D.  3D (still) and 4D (moving) ultrasound pictures can be taken to visualize details of the anatomy (face, brain, and spine)


Fetal Echocardiogram


Uses ultrasound waves to assess details of the baby's heart anatomy and function. This is used to help assess suspected congenital heart defects.


Fetal echocardiograms, Doppler and color flow mapping medically necessary for any of the following conditions:

  • A mother with insulin dependent diabetes mellitus or systemic lupus erythematosus

  • As a screening study in families with a first-degree relative with a history of congenital heart disease

  • Fetal nuchal translucency measurement of 3.5 mm or greater in the first trimester

  • Following an abnormal or incomplete cardiac evaluation on an anatomic scan, 4-chamber study.

  • For ductus arteriosus dependent lesions and/or with other known complex congenital heart disease

  • For pregnancies conceived by in vitro fertilization (IVF) or intra-cytoplasmic sperm injection (ICSI)

  • In cases of single umbilical artery

  • In cases of suspected or known fetal chromosomal abnormalities

  • In suspected or documented fetal arrhythmia: to define the rhythm and its significance, to identify structural heart disease and cardiac function

  • In members with autoimmune antibodies associated with congenital cardiac anomalies [anti-Ro (SSA)/anti-La (SSB)]

  • In members with familial inherited disorders associated with congenital cardiac abnormalities (e.g., Marfan syndrome)

  • In cases with monochorionic twins or multiple gestation and suspicion of twin-twin transfusion syndrome

  • In members with seizure disorders, even if they are not presently taking anti-seizure medication

  • In cases with non-immune fetal hydrops or unexplained severe polyhydramnios

  • When members' fetuses have been exposed to drugs known to increase the risk of congenital cardiac abnormalities including but not limited to:

    Anti-seizure medications

    Excessive alcohol intake


    Paroxetine (Paxil)


  • When other structural abnormalities are found on ultrasound



Repeat studies of fetal echocardiograms medically necessary when the initial screening study indicates any of the following:

  • A ductus arteriosus dependent lesion; or

  • Structural heart disease with a suggestion of hemodynamic compromise; or

  • Tachycardia other than sinus tachycardia or heart block.

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