Fetal Ultrasound

Why It Is Done

Fetal ultrasound is done to learn about the health of your fetus. Different details can be learned at different times during your pregnancy.

• 1st-trimester fetal ultrasound is done to:
  • See how your pregnancy is going.
  • Find out if you are pregnant with more than one fetus.
  • Estimate the age of the fetus (gestational age).
  • Estimate the risk of a chromosome defect, such as Down syndrome.
  • Check for birth defects that affect the brain or spinal cord.
• 2nd-trimester fetal ultrasound is done to:
  • Estimate the age of the fetus.
  • Look at the size and position of the fetus, placenta, and amniotic fluid.
  • Check the position of the fetus, umbilical cord, and placenta during a procedure such as amniocentesis or umbilical cord blood sampling.
  • Find major birth defects, such as a neural tube defect or heart problems.
• 3rd-trimester fetal ultrasound is done to:
  • Make sure that a fetus is alive and moving.
  • Look at the size and position of the fetus, placenta, and amniotic fluid.

Transvaginal ultrasound is most often done early in a pregnancy to check the age of the fetus. It may also be done if your doctor thinks you may have an ectopic pregnancy. It is sometimes done late in pregnancy to find the location of the placenta. And it may be done in a high-risk pregnancy to check the length of the cervix.

How To Prepare

You may need a full bladder for the test. If so, you will be asked to drink water or other liquids just before the test. You will be asked not to urinate before or during the test. In most cases, women in the third trimester do not need to have a full bladder.

For a transvaginal fetal ultrasound, the vaginal transducer is usually covered with a latex sleeve and a lubricant, such as K-Y Jelly. If you are allergic to latex, tell the health professional before you have the test.

Talk to your doctor if you have any concerns about the need for the ultrasound, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is Done

Most often, a fetal ultrasound is done by an ultrasound technologist. But it may be done by a radiologist or obstetrician. Fetal ultrasound can be done in a doctor's office, hospital, or clinic.

You may not need to remove your clothes for this test. You can lift your shirt and push down your skirt or pants. If you are wearing a dress, you will get a cloth or paper covering to use during the test.

Transabdominal ultrasound

• You may need to have a full bladder. A full bladder helps transmit sound waves, and it pushes the intestines out of the way of the uterus. This makes the ultrasound picture clearer.
  • You will not be able to urinate until the test is over. But tell the ultrasound tech if your bladder is so full that you are in pain.
  • If an ultrasound is done during the later part of pregnancy, a full bladder may not be needed. The growing fetus will push the intestines out of the way.
• You will lie on your back on an exam table. If you become short of breath or feel faint while lying on your back, your upper body may be raised or you may be turned on your side.
• A gel will be spread on your belly.
• A small, handheld device called a transducer will be pressed against the gel on your skin. It will be moved across your belly several times. You may watch the monitor to see the picture of the fetus during the test.

The test takes about 30 to 60 minutes. When it is finished, the gel is cleaned off your skin. You can urinate as soon as the test is done.

Ultrasound techs are trained to gather images of your fetus. But they can't tell you if it looks normal or not. Your doctor will share this information with you after the ultrasound images have been reviewed by a radiologist or perinatologist.

Transvaginal ultrasound

• You do not need to have a full bladder.
• You will lie on your back with your hips slightly raised.
• A cover (such as a condom) will be placed over the thin transducer. The transducer will be put gently into your vagina. It will be moved to adjust the image on the monitor. Some doctors may let you to put the transducer into your vagina yourself.

This test takes about 15 to 30 minutes.

How It Feels

During a transabdominal ultrasound, you may have a feeling of pressure in your bladder. The gel may feel cool when it is first put on your belly. You will feel a light pressure when the transducer is passed over your belly.

Normally a transvaginal ultrasound does not cause discomfort. You may feel a light pressure when the transducer is moved in your vagina.

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

• You are very overweight.
• You have stool (feces) or air in your intestines or rectum.
• You have an abnormally low amount of amniotic fluid.
• Your fetus is in certain positions.
• You are not able to lie still during the test.
• The fetus is very active.

What To Think About

• Fetal ultrasounds do not always show birth defects. Normal ultrasound results can't guarantee that your baby will be healthy. No test can do that.
• Your doctor may suggest more tests if the results of your fetal ultrasound are not normal.
• A photograph or videotape of the ultrasound image of the fetus may be offered to you.
• Your due date may be changed based on an ultrasound done in early pregnancy. The ultrasound may predict a different date, based on fetal size and development.
• In the third trimester, fetal ultrasound may not find the exact fetal age or weight.
• The effects of prolonged ultrasound exposure on the fetus have not been studied. So the U.S. Food and Drug Administration (FDA) does not recommend fetal ultrasound for nonmedical reasons. Examples are finding out the sex of the fetus or getting a keepsake image.
• Three-dimensional (3-D) fetal ultrasound is being tested for use in finding fetal abnormalities. It is not yet widely available.
• Doppler ultrasound (or duplex scanning) uses reflected sound waves to estimate the speed and direction of blood as it flows to the placenta and within the fetus. For more information, see the topic Doppler Ultrasound.