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Obstetric Ultrasonography - Pregnancy Ultrasound

Overview & Description

An ultrasound is something like an x-ray. But it uses sound waves rather than radiation to make black-and white pictures from inside the body. A hand-held device called a transducer sends high-frequency sound waves through the body. The sound waves echo off of body structures. A computer converts the echoes into visual images. An ultrasound allows a healthcare provider to view a pregnant woman's organs and the growing baby.

A transvaginal ultrasound might be done during the first three months of pregnancy. For this test, the transducer is placed in the vagina. An abdominal ultrasound might be done later in pregnancy. For the abdominal ultrasound, the transducer is placed on the abdomen.

Who is a candidate for the test?

An ultrasound is used in women who are pregnant, or who might be pregnant. An ultrasound might be done more than once during a pregnancy, depending on the health of the baby or mother.

A healthcare provider uses an ultrasound for many reasons, such as to:

  • confirm the expected date of a baby's birth
  • look for size or placement problems with the placenta
  • evaluate causes of vaginal bleeding, such as a blighted ovum, which is a fertilized egg that has stopped growing
  • rule out ectopic pregnancy, a condition in which the fertilized egg grows outside of the uterus
  • check for intrauterine growth retardation, which occurs when the baby grows too slowly
  • evaluate the volume of amniotic fluid
  • assess enlarged ovaries
  • diagnose multiple pregnancies, such as twins or triplets
  • rule out molar pregnancy, a situation in which the fetus itself becomes a tumor
  • check for problems with the baby, including spina bifida, where the spine fails to close during development, or cleft palate, which is abnormal closure of the lip and roof of the mouth
  • determine if the baby is alive and healthy, with good movement, heart function, and placement in the uterus
  • An ultrasound is used during amniocentesis to determine the baby's position in the womb. In this procedure, a needle is placed through the abdomen to withdraw a sample of the fluid around the baby. It is also used during chorionic villus biopsy. With this test, the doctor takes a sample of the membrane around the baby and womb.

    How is the test performed?

    An ultrasound is something like the sonar used in ships at sea or by bats. As sound waves bounce off objects or organs, they can show location, size, and shape of the organ. The equipment consists of:

  • a transducer, which is a small hand-held device that looks like a microphone
  • a screen to view the images that is like a television or computer monitor
  • Liquid jelly is used on the transducer to help transmit the sound waves. The transducer both sends the sound waves and records the echoes. A computer helps translate the echoes into images. These pictures can be recorded both as video and as still images.

    For the test, the woman must lie on her back or side. The lower abdomen is exposed. The transducer is moved across the outside of the abdomen. The ultrasound should take about 20 minutes to complete.

    During the first three months, a probe called an endotransducer may be used instead. This probe is placed in the vagina. It can provide better images than the abdominal method in the early stages of pregnancy.

    The healthcare provider may also use a Doppler ultrasound. This special type of ultrasound measures blood flow in the blood vessels. It checks movement in organs. One reason to use a Doppler ultrasound is to check the movement of heart valves.

    Preparation & Expectations

    What is involved in preparation for the test?

    The woman should wear loose-fitting, two-piece clothes. If done early in the pregnancy, the woman may need to have a full bladder. This can help with getting a clearer picture from the sound waves. A full bladder is not usually needed in the later stages of pregnancy.

    Results and Values

    What do the test results mean?

    A pregnancy ultrasound can show if the baby is growing in a healthy manner with no visible problems. It shows that the baby is in a healthy position. It also shows that the mother has a healthy-appearing placenta with high enough amniotic fluid levels.

    Healthy ultrasound results do not guarantee a perfect baby or an uncomplicated birth. But it can be helpful in telling the healthcare provider how a woman's pregnancy is progressing.

    Abnormal results vary. A woman should discuss the results and any treatments with her healthcare provider.


    Author:Eva Martin, MD
    Date Written:
    Editor:Crist, Gayle P., MS, BA
    Edit Date:10/17/02
    Reviewer:Kathleen A. MacNaughton, RN, BSN
    Date Reviewed:10/17/02