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Ultrasound in Pregnancy

What is an ultrasound?

 Picture of a physician performing an ultrasound on an expectant mother

An ultrasound scan is a diagnostic technique which uses high-frequency sound
waves to create an image of the internal organs. A screening ultrasound is sometimes done during the course of a pregnancy to monitor normal fetal growth and verify the due date. Results of ultrasounds from the first 14 weeks are most accurate in establishing or confirming due dates. Ultrasounds may be done at various times throughout pregnancy for different reasons:

In the first trimester:

  • To establish the dates of a pregnancy

  • To determine the number of fetuses and identify placental structures

  • To diagnose an ectopic pregnancy or miscarriage

  • To examine the uterus and other pelvic anatomy

  • In some cases to detect fetal abnormalities

Mid-trimester: (sometimes called the 18 to 20 week scan)

  • To confirm pregnancy dates

  • To determine the number of fetuses and examine the placental structures

  • To assist in prenatal tests such as an amniocentesis

  • To examine the fetal anatomy for presence of abnormalities

  • To check the amount of amniotic fluid

  • To examine blood flow patterns

  • To observe fetal behavior and activity

  • To examine the placenta

  • To measure the length of the cervix

  • To monitor fetal growth

Third trimester:

  • To monitor fetal growth

  • To check the amount of amniotic fluid

  • As part of a biophysical profile

  • To determine the position of a fetus

  • To assess the placenta

How is an ultrasound scan done?

Although the specific details of each procedure vary slightly, generally, ultrasounds follow this process. Two types of ultrasounds can be done during pregnancy:

  • Abdominal ultrasound. In an abdominal ultrasound, gel is applied to the abdomen and the ultrasound transducer glides over the gel on the abdomen to create the image. A woman may need to have a full bladder for abdominal ultrasounds in early pregnancy.

  • Transvaginal ultrasound. In a transvaginal ultrasound, a smaller ultrasound transducer is inserted into the vagina and rests against the back of the vagina to create an image. A transvaginal ultrasound produces a sharper image and is often used in early pregnancy.

There are several types of ultrasound imaging techniques. The most common is two dimensional, or 2D. This gives a flat picture of one aspect of the image.

If more information is needed, a 3D ultrasound exam can be done. This technique, which provides a three-dimensional picture, requires a special machine and special training. But the 3D image allows the doctor to see width, height, and depth of images, which can be helpful in diagnosis. The 3D images can also be captured and saved for later review.

The latest technology is 4D ultrasound, which allows the health care provider to visualize the unborn baby moving in real-time. With 4D imaging, a three-dimensional image is continuously updated, providing a "live action" view. These images often have a golden color, which helps show shadows and highlights.

Ultrasound images may be captured in still photographs or on video to document findings.

What are the risks and benefits of ultrasound?

Fetal ultrasound has no known risks other than mild discomfort due to pressure from the transducer on the abdomen or in the vagina. No radiation is used during the procedure.

Transvaginal ultrasound requires covering the ultrasound transducer in a plastic or latex sheath, which may cause a reaction in people with a latex allergy.

Fetal ultrasound is sometimes offered in nonmedical settings to provide keepsake images or videos for parents. While the ultrasound procedure itself is considered safe, it is possible that untrained personnel may give parents false assurances about their baby's well-being, or perhaps an abnormality may be missed. Having an ultrasound done by trained medical personnel who can correctly interpret findings is recommended. Talk with your health care provider if you have questions.

Ultrasound is a technique that is constantly being improved and refined. As with any test, results may not be completely accurate. However, ultrasound can provide valuable information for parents and health care providers to help manage and care for the pregnancy and fetus. In addition, ultrasound gives parents a unique opportunity to see their baby before birth, helping them to bond and establish an early relationship.

Online Medical Reviewer: Dozier, Tennille, RN, BSN, RDMS
Online Medical Reviewer: newMentor board-certified, academically affiliated clinician
Date Last Reviewed: 12/30/2013
© 2000-2014 The StayWell Company, LLC. 780 Township Line Road, Yardley, PA 19067. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.