Ultrasound Scanning in Pregnancy

What is ultrasound scanning?

Ultrasound scanning is a procedure that uses high-frequency sound waves and their echoes to take pictures of structures inside the body. It does not use X-rays and is very safe for you and the baby.

When is it used in pregnancy?

Your healthcare provider uses ultrasound scanning to examine you and your pregnancy. It is used to look at the baby, the placenta, the fluid around the baby, your uterus (womb), your ovaries, and your cervix (the opening of the womb into the vagina).

You may have 1 or more ultrasound scans as part of your routine prenatal care. You may have additional scans if your healthcare provider thinks there may be a problem with the pregnancy or if you have risk factors that could lead to problems for the baby. Ultrasound scanning is very helpful for many abnormal conditions.

Some common reasons ultrasound scanning may be done during pregnancy are to:

• See how far along you are in your pregnancy and to confirm a due date.
• Check the age, size, weight, and position of the baby.
• Make sure the baby is developing in the uterus and not in a fallopian tube (ectopic pregnancy).
• See if the placenta is normal and attached properly.
• Look at the baby’s behavior, such as movements and practice breathing.
• Help with an amniocentesis or other tests or procedures.
• Check for more than 1 baby.
• Check the baby's heart.
• Check the amount of fluid around the baby inside the uterus.
• Check the thickness of the skin on the baby’s neck to help check for birth defects. This measurement is called a nuchal translucency.
• Check the baby for signs of stress as part of a biophysical profile.
• Check for any problems with the uterus, such as an unusual shape or fibroids.
• See if the baby's body has any major problems that might suggest a birth defect or genetic problem.
• Check into the cause of vaginal bleeding or pelvic pain.

An ultrasound scan may be done for different reasons at different stages of the pregnancy. For example:

• Ultrasound exams in the first or early second trimester (before 16 weeks) are very helpful to check the age of the baby, confirm the due date and check for more than 1 baby.
• Exams between 18 and 22 weeks are often used to check the baby for any signs of a genetic problem or birth defect.
• Tests to see if the baby is growing at a normal rate are most accurate during the late second and third trimester of pregnancy.

Sometimes an ultrasound can tell whether the baby is a boy or a girl after 16 weeks of pregnancy. However, this is not usually the main reason for doing an ultrasound.

How do I prepare for ultrasound scanning?

Most of the time you do not need to have a full bladder to have an ultrasound exam. However, every once in a while a full bladder is needed to see some structures in more detail. For this reason your healthcare provider may ask you to drink up to 6 glasses of water 1 hour before the test and then not to urinate until after the test. Your provider will tell you if this or other preparations are needed.

What happens during the procedure?

The ultrasound scan may be done at your healthcare provider's office, at a clinic or hospital, or at a radiology center.

A small device called a transducer probe is used to do the scan. The transducer acts like a microphone. During the scan, sound waves enter your body through the transducer. (They are not the type of sound waves that you can hear.) The transducer is connected to a computer. As the sound waves pass through your body, they are reflected back to the transducer by structures in the body, such as your uterus or the baby. The computer analyzes these echoes and makes images of the baby and your organs on a screen.

The ultrasound exam may be done by putting the transducer on the surface of your abdomen. First your healthcare provider or the ultrasound technologist will put an oil or gel on the skin of your belly. This helps the transducer glide smoothly over your skin and improves its contact with your body. Your provider will move the transducer over different parts of your belly to get pictures of the baby and the uterus. You may be able to watch the pictures of your baby on the screen during the exam.

Some ultrasound exams, especially early in pregnancy, are done from inside the vagina. For these scans the transducer is shaped like a narrow tube and covered with a protective cover that is like a condom. It is inserted gently into the vagina using a small amount of gel as a lubricant.

An abdominal ultrasound scan may take as long as an hour. A vaginal ultrasound is much quicker and usually takes less than 5 minutes.

What happens after the procedure?

Usually you can go home and go back to your normal activities as soon as the scan is done. You may get the results of the ultrasound within a few minutes to a few days later.

What are the benefits of this procedure?

• Ultrasound scans can improve a baby's chances of health and survival by:
  • finding problems that might be corrected before the baby is born
  • giving more detailed information about the baby's health before birth
  • allowing better dating of the pregnancy to know when the baby is due (this can help prevent premature birth if there is a question of needing to deliver the baby early)
  • giving information that can help determine if the baby needs to be delivered early.
• There are no known side effects from ultrasound scanning.
• No medicines are known to interfere with test results.
• The test is fast and painless.
• The test does not use radiation.
• The test avoids possible problems of some other tests, such as bleeding, infection, or reactions to chemicals.

What are the risks of this procedure?

Some women may feel a little discomfort from the pressure of the ultrasound transducer. However, there are no known physical risks associated with ultrasound scanning because the sound waves are not dangerous.

Although ultrasound can tell us many things about a pregnancy, it is not a perfect test. It is possible for the baby to have a problem that is not found with the exam. It is also possible for the test to suggest a problem when the baby is normal. Some things that might affect the accuracy of the exam are:

• the baby’s position
• the baby’s age
• the amount of fluid around the baby
• a mother who is very overweight
• extra tissue in the mother’s abdomen that prevents sound waves from passing easily, making it hard to take clear pictures.

When should I call my healthcare provider?

Call your provider during office hours if:

• You have questions about the procedure or its result.
• You want to make another appointment.