Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasonography in pregnancy to provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus.


  • It produces pictures of the inside of the body using sound waves.
  • It uses a small probe called a transducer and gel placed directly on the skin.
  • High-frequency sound waves travel from the probe through the gel into the body. The probe collects the sounds that bounce back.
  • A computer uses those sound waves to create an image.  Because images are captured in real-time, they can show the structure and movement of the body’s internal organs.
  • They can also show blood flowing through blood vessels.


  • Doppler ultrasound is a special ultrasound technique that evaluates movement of materials in the body.
  • It allows the doctor to see and evaluate blood flow through arteries and veins in the body.
  • During an obstetrical ultrasound the examiner may evaluate blood flow in the umbilical cord or may, in some cases, assess blood flow in the fetus or placenta.


– Trans Abdominal scan (TAS):

Traditional obstetric sonograms are mostly done by placing a transducer on the abdomen of the pregnant woman.


-Transvaginal scan (TVS):

 Transvaginal sonography, is done with a probe placed in the woman’s vagina. Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women.


-Doppler sonography:   

which detects the heartbeat of the fetus. Doppler sonography can be used to evaluate the pulsations in the fetal heart and bloods vessels for signs of abnormalities.


-3D & 4D ultrasound :

Main article: 3D ultrasound

Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology.



  • A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks’ gestational age (approximately 3 weeks after ovulation). The embryo should be seen by the time the gestational sac measures 25 mm, about five-and-a-half weeks.
  • The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 7 mm, around 7 weeks’ gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly after normal heartbeat is detected.

In the first trimester, a standard ultrasound examination typically includes:

• Gestational sac size, location, and number
• Identification of the embryo and/or yolk sac
• Measurement of fetal length (known as the crown-rump length)
• Fetal number, including number of amniotic sacs and chorionic sacs for multiple gestations
• Embryonic/fetal cardiac activity
• Assessment of embryonic/fetal anatomy appropriate for the first trimester
• Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures.
• NT Scan-Evaluation of the fetal nuchal fold, with consideration of fetal nuchal translucency assessment


In the second trimester, a standard ultrasound exam typically includes:

  • Fetal number, including number of ammonic sacs and chorionic sacs for multiple gestations
  • Fetal cardiac activity
  • Fetal position relative to the uterus and cervix
  • Location and appearance of the placenta, including site of umbilical cord insertion
  • Ammonic fluid volume
  • Gestational age assessment
  • Fetal weight estimation
  • Fetal anatomical survey
  • Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures when appropriate.

Dating and Growth Monitoring 

  • Gestational age is usually determined by the date of the woman’s last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle.
  • Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle.
  • Ultrasound scans offer an alternative method of estimating gestational age. The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation.
  • After 13 weeks of gestation, the fetal age may be estimated using the biparietal diameter , the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters.
  • Dating is more accurate when done earlier in the pregnancy; if a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate.
  • The abdominal circumference of the fetus may also be measured. This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth.

Cervical Length & Premature Birth –

  • Obstetric sonography is useful in the assessment of the cervix in women at risk for premature birth.
  • A short cervix preterm is associated with a higher risk for premature delivery
  • At 24 weeks’ gestation, a cervix length of less than 25 mm defines a risk group for spontaneous preterm birth.
  • Further, the shorter the cervix, the greater the risk. Cervical measurement on ultrasound also has been helpful to use ultrasonography in patients with preterm contractions, as those whose cervical length exceeds 30 mm are unlikely to deliver within the next week.


  • Routine pregnancy sonographic scans are performed to detect developmental defects before birth.
  • This includes checking the status of the limbs and vital organs, as well as (sometimes) specific tests for abnormalities.
  • Some abnormalities detected by ultrasound can be addressed by medical treatment in utero or by perinatal care, though indications of other abnormalities can lead to a decision regarding abortion.


  • The most common test to detect any fetal anomaly is to measure of the nuchal translucency thickness (“NT-test”, or “Nuchal Scan”).
  • Ultrasound is done between 11- and 13.6-weeks pregnancy.
  • Nuchal translucency (NT) has emerged as the most sensitive ultrasound marker for detection of chromosomal anomalies in the first trimester
  • Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome.


  • Ultrasound may also detect fetal organ anomaly.
  • Usually scans for this type of detection are done around 18 to 23 weeks of gestational age called the “anatomy scan”, “anomaly scan,” or “level 2 ultrasound”
  • Anomaly scans are also clearly beneficial because ultrasound enables clear clinical advantages for assessing the developing fetus in terms of morphology, bone shape, skeletal features, fetal heart function, volume evaluation, fetal lung maturity and general fetus well being.


  • Second-trimester ultrasound screening for aneuploidies is based on looking for soft markers and some predefined structural abnormalities
  • Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones. These markers are often not clinically significant and do not cause adverse pregnancy outcomes.


  • Ultrasound is safe and painless.
  • Ultrasound exams do not use radiation (as used in x-rays).
  • Current evidence indicates that diagnostic ultrasound is safe for the unborn child, unlike radiographs, which employ ionizing radiation.
  • Doppler ultrasonography examinations have a thermal index (TI) of about five times that of regular (B-mode) ultrasound examinations. so, to be used cautiously.
  • There is usually no discomfort from pressure as the transducer is pressed against the area being examined. However, if scanning is performed over an area of tenderness, you may feel pressure or minor pain from the transducer.


  • This ultrasound examination is usually completed within 30 minutes.
  • When the exam is complete, you may be asked to dress and wait while the ultrasound images are reviewed.
  • After an ultrasound examination, you should be able to resume your normal activities immediately.


  • Obstetric ultrasound cannot identify all fetal abnormalities.
  • Consequently, when there are clinical or laboratory suspicions for a possible abnormality, a pregnant woman may have to undergo nonradio logic testing such as a blood test or amniocentesis (the evaluation of fluid taken from the sac surrounding the fetus) or chorionic villus sampling (evaluation of placental tissue) to determine the health of the fetus, or she may be referred by her primary care provider to a primatologist (an obstetrician specializing in high-risk pregnancies).
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