Like other ultrasound examinations, prenatal or fetal ultrasound uses high-frequency sound waves to obtain images of the fetus, placenta, and amniotic sac in a pregnant woman. The test does not involve radiation and is completely safe for both mother and baby. An ultrasound examination is also known as a sonogram. The entire procedure can be performed in 30 minutes or less. Two types of ultrasound exams are used in pregnancy.

• Transabdominal ultrasound

Two types of ultrasound exams are used in pregnancy. The most common is a transabdominal ultrasound, in which the measuring instrument (transducer) is moved over the surface of the abdomen after a gel has been applied. Sometimes a full bladder is required to obtain better images, so you may be asked to drink several glasses of water before the test. There is no pain or discomfort associated with the test, and it can be performed in the doctor's office.

• Transvaginal ultrasound

Transvaginal ultrasound uses a probe that is inserted directly into the vagina. It is performed in the doctor's office similar to a pelvic examination. This type of exam is most commonly used in the early weeks of pregnancy to rule out suspected problems or to assess the gestational age of the embryo. In early pregnancy this examination can provide more accurate information then a transabdominal examination.

As mentioned above, transvaginal ultrasound is most often used in the early weeks of pregnancy to rule out problems or to determine how far along you are in the pregnancy. Most pregnant women receive a transabdominal ultrasound around the 20th week of pregnancy. This exam confirms that the baby is growing normally and that the placenta is attached normally. The heartbeat of the fetus is visible, and the movement of the fetus can be observed. Major birth defects can be visualized by this method, as well. In most cases, it is possible to determine the sex of the baby through an ultrasound exam at 20 weeks, but this method is not 100% accurate. You can tell the examiner whether or not you wish to know the gender of your baby at the time of the examination.

Ultrasound may also be performed earlier or later in the pregnancy for specific reasons that include:

• Determination of multiple gestation
• Ensuring the health of the baby and monitoring its growth
• Determining the location of the placenta
• Estimation of gestational age and due date
• Assessing the expected size and weight of the baby
• Determining the amount of amniotic fluid
• Determining the position of the baby

A 3D ultrasound provides a particularly clear image that resembles a photograph. A so-called 4D ultrasound provides this image in real time. Sometimes these ultrasound images are offered by non-medical providers in stores or other locations. Medical authorities, including The American College of Obstetricians and Gynecologists and the Food and Drug Administration do not recommend using these services because the examiners may not have received proper training and could provide inaccurate information.

As mentioned before, there is no radiation exposure during a prenatal ultrasound. Ultrasound exams have been used for many years, and studies have never shown any harms associated with the procedure, either on a short- or long-term basis.