The ultrasonic image on the computer screen. Image credit: Flickr
The ultrasonic image on the computer screen. Image credit: Flickr

Obstetric ultrasound (also called pregnancy sonogram, obstetric sonogram, prenatal ultrasound and obstetric ultrasonography) is a procedure that refers to usage of ultrasound scans during pregnancy. During the procedure, an equipment called real-time scanner (transducer) is positioned over the pregnant woman’s stomach, resulting in a series of continuous pictures that depict the moving fetus. Movements like fetal heart beat malformations (if any) can be detected and assessed.

The procedure helps determine the fetus’ gestational age, growth and size. The total number of obstetric ultrasound sessions vary across women, and the actual number depends on the problems detected in the previous scan and the follow-ups needed, as a result. And since there’s no radiation involved, the process is safe.

During the test, a gel-like substance is applied on the area to help the scanner glide over the spot and ensure zero air pockets between the skin and scanner. Generally, the sound wave frequency used is in the 3.5 to 7.0 megahertz range. Though the sound waves from the transducer cannot be felt, the gel may feel a bit wet and cold.

Test Preparation

The procedure doesn’t require any major preparation. The pregnant woman’s bladder must be full when the procedure is carried out during early pregnancy. This helps cancel out the presence of air in the abdomen, which may interfere with the sound waves. Generally, two to three glasses of water are recommended an hour prior to the test. The woman shouldn’t urinate during the one hour. Two-piece, loose-fitting outfit must be worn for the test, without jewelries and other accessories. The second and third trimester tests don’t require a full bladder.

Test Procedure

The ultrasound is a simple, painless procedure that doesn’t last more than 30 minutes. During the procedure, the pregnant woman lays flat on her back, with her belly exposed. As aforementioned, the gel-based solution is smeared over her abdomen. The operator then places the transducer on the belly and sets it into motion to capture fetus images.

The examination in progress. Image credit: Wikimedia Commons
Obstetric ultrasound examination in progress. Image credit: Wikimedia Commons


The procedure is primarily carried out to determine any pregnancy issues, overall progress and also to chart out proper prenatal care. The ultrasound is performed during the first trimester to:

  • Confirm a pregnancy
  • Ascertain baby’s age
  • Find out potential issues, like ectopic pregnancies or any miscarriage possibilities
  • Determine the heart rate of the baby
  • Check for multiple birth possibilities (for instance, twins or triplets)
  • Spot issues of the uterus, placenta, ovaries and cervix

The ultrasound is carried out during 2nd and 3rd trimesters to:

  • Determine baby age, position, growth and also gender, at times
  • Detect any issues with fetus development
  • Look for multiple birth possibilities
  • Observe the amniotic fluid, pelvis and placenta

Procedure Accuracy Issues or Limitations

Since there’s a manual element involved in obstetric ultrasounds, the actual diagnosis and results of the technique may vary with different scans/operators. The more experienced the physician/operator, the more insightful and comprehensive is the test. For instance, an apparent birth defect may not be detected during a particular ultrasound test. And in another test, a fetal abnormality may get reported when there wasn’t one at all.

A transvaginal ultrasound, instead of a transabdominal ultrasound, is often preferred during early pregnancy. This is because the probe could then get much closer to the uterus and visualize the fetus better. After the first trimester, a transabdominal ultrasound offers more benefits. For increased accuracy, a blood test is often done along with the ultrasound.