What is an Ultrasound?
Ultrasound (US) imaging is a method of obtaining images of internal organs in the body by sending high-frequency sound waves into the body. The image itself is the sound wave echoes of the ultrasound. As the sound waves bounce against objects, its echoing waves can be used to identify how far away the object is, how large it is, its shape and its internal consistency, be it fluid, solid or mixed. These sound waves are instantly measured and displayed on a computer as a video or still picture.
A benefit of ultrasound is that no ionizing radiation is involved, unlike conventional x-rays. In pregnancy, obstetric ultrasound uses sound waves to visualize and determine the condition of a pregnant woman and her embryo or fetus.
Why Would I Have an Ultrasound?
There are several indications for ultrasound in a pregnant woman including:
- Establishing the presence of a living embryo or fetus
- Determine the cause of bleeding in early or mid-late pregnancy
- To diagnose any potential congenital abnormalities in the developing embryo or fetus. If birth defects are suspected, your provider will refer you for a more detailed ultrasound examination.
- To determine if there are twins or multiple pregnancies
- To determine the location, size or possible abnormalities of the placenta.
- To estimate the age and size of the fetus. Size of the fetus is important when preterm delivery is being contemplated or when the baby is believed to be late.
- To evaluate the position of the fetus and the placenta and to locate the fetus prior to chorionic villus sampling or amniocentesis
- Determine the condition of the fetus if no heartbeat has been detected by 14th week or if there has been on fetal movement by week 22.
- Locate an IUD that was in place at the time of conception.
- Measure the amount of amniotic fluid
- Check for fibroids within the uterus.
- Detect cervical changes that might predict preterm labor.
- Verify breech presentation or other uncommon fetal or cord position before delivery.
- Providing valuable information leading to treatment that can improve a woman’s chances of having a healthy baby.
Preparing For the Ultrasound
Your doctor will explain that you should wear loose fitting clothes for the examination. Only the lower abdominal area needs to be exposed during this procedure, so wear something that can be easily removed. You may be required to have a full bladder because this will push the air-filled bowel out of the way and allow an unobstructed view of the uterus, embryo or fetus to be obtained. You may be instructed to drink up to six glasses of water and avoid urinating during the two hours before the procedure. If you are having the ultrasound later on in your pregnancy, a full bladder is generally not required.
A level 1 ultrasound is usually done to date a pregnancy and is performed before 12 weeks. A more detailed (level 2) ultrasound is used for more sophisticated diagnostic procedures and is normally done between week 18 and 22. A benefit of ultrasound is that it is a painless procedure. The only discomfort that you may feel would be when the radiologist guides a hand-held device (called a transducer) over your abdomen. Occasionally, a device may need to be inserted into your vagina but again pain is not common, only discomfort. At times, the ultrasound devise may have to be pressed more firmly to get closer to the embryo or fetus to better visualize them.
The exam typically takes about 20 minutes. During the exam you will lie on your back. A film gel is applied to the abdomen to improve the conduction of sound. A transducer is then moved slowly over the abdomen or inserted into the vagina, and the instruments record echoes of sound waves as they bounce off parts of the baby and translates them into pictures. A medical radiologist will then analyze the images that were produced during the ultrasound and send a signed report with his or her interpretation to the patient’s personal physician. You may be able to differentiate the beating heart, the curve of the spine, or the head, arms and legs.
You will receive the results directly from the referring physician who ordered the tests initially. In some cases, you may receive a “photo” of your baby, a videotape, or a three-dimensional image that you can show to your friends and family. If you don’t want to know your baby’s sex in advance, be sure to let the doctor or technician know as sometimes the genital organs are distinguishable and the sex can be surmised with ultrasound.
Risks and Benefits of Ultrasound
Ultrasound is safe for you and your baby because it does not use x-rays to produce an image. Using sound waves so high they can’t be heard by the human ear, ultrasound allows visualization and “examination” of the fetus without X-rays. Studies show that low-risk women who have routine ultrasound exams are no more likely to have healthy babies than low-risk women how don’t have the test. The only potential risk is the chance that ultrasound may not detect all birth defects, and occasionally give a false positive result, where the patient is told incorrectly that her baby may have a birth defect. Frequent follow-up exams can ensure that false positive results are kept to a minimum.
Revision date: June 18, 2011
Last revised: by Janet A. Staessen, MD, PhD