What is a fetal ultrasound?
Fetal ultrasound is a test used during pregnancy that creates an image of the fetus in the mother’s uterus, or womb. During a fetal ultrasound, various parts of the baby, such as the heart, head, and spine, are identified and measured. The testing may be performed either through the mother’s abdomen (transabdominal) or vaginal canal (transvaginal). The health of an unborn baby can be readily evaluated using fetal ultrasound.
There are several types of fetal ultrasound, each with a specific advantage in certain situations. A Doppler ultrasound, for example, helps to study the movement of blood through the umbilical cord between the uterus and placenta. Three-dimensional ultrasound provides a life-like image of an unborn baby. The standard, two-dimensional ultrasound is the primary focus of this discussion, but the basic concepts for standard fetal ultrasound also apply to the other types.
How does ultrasound work?
Ultrasound uses an electronic device called a transducer to send and receive sound waves. When the transducer is moved over the abdomen, the ultrasonic sound waves then move through the skin, muscle, bone, and fluids at different speeds. The sound waves bounce off the fetus like an echo, returning to the transducer. The transducer picks up the reflected waves and converts them into an electronic picture.
A clear gel is placed between the transducer and the skin to allow for the best sound conduction and smooth movement of the transducer.
Fetal abnormalities are relatively uncommon and depend upon a variety of factors relating to the health and age of the mother as well as other genetic and environmental variables. In order to identify and potentially correct any problems, certain fetal structures are checked during routine ultrasonography. The American College of Radiology and the American Institute of Ultrasound in Medicine suggest that the following structures should be evaluated during a routine prenatal ultrasound:
- 1head and brain – the chambers within the brain (ventricles), distance between parietal bones of the fetal head (biparietal diameter), and bone depression at the back of head (nuchal area) are evaluated for defects.
- heart – the chambers and valves of the heart are evaluated and defects may be identified.
- abdomen and stomach – the size, location, and arrangement of intestines are checked.
- urinary bladder – the size and presence of the bladder is evaluated.
- spine – defects may be identified if present.
- umbilical cord – three blood vessels should be attached at the front of the abdomen.
- kidneys – two kidneys should be present on either side of the mid-spine.
other fetal structures – limbs and other parts may also be scanned and evaluated.
Reasons for the Procedure
Fetal ultrasound has become a routine part of prenatal care in most of the US. This is due to the fact that it is a low risk procedure that provides valuable information with relative ease. General progress during a pregnancy can be efficiently evaluated using ultrasound. For example, placement of the placenta, the organ linking the blood supply between mother and baby, is checked to make sure the fetus will receive nutrients needed for growth.
Multiple birth pregnancies carry a higher risk than single birth pregnancies, and ultrasound allows for confirmation of multiples. Likewise, accurate and safe examination of each fetus is made possible with fetal ultrasound.
Determining the gestational age of a baby is another use for fetal ultrasound. In the first trimester, gestational age is generally calculated by measuring the length from the crown of the baby’s head to its rump. In the second and third trimesters, the most accurate way to determine gestational age is by measuring the baby’s head circumference and femur (thigh bone) length.
Nuchal translucency screening also involves using fetal ultrasound. Nuchal refers to the back of the neck, an area where cerebrospinal fluid may accumulate in an unborn baby. As part of routine screening, this area is checked to see if there is an increase in fluid or thickening. The screening occurs sometime near the tenth and fourteenth weeks of pregnancy. When combined with blood tests, this screening can help to determine the risk of certain birth defects.
False-positive test results may occur, however, indicating a problem when the fetus is actually healthy. Likewise, false-negative test results may indicate a normal result when the fetus actually does have a health problem. Additional testing may be needed if there is doubt regarding test results; however, the chance of inaccuracy is slight due to continual improvements in technology and training.
If an amniocentesis is done, a fetal ultrasound may be used to help with placement of the needle, which is used to remove a sample of the amniotic fluid surrounding the fetus.
Risks of the Procedure
Fetal ultrasound has no known risks other than mild discomfort due to pressure from the transducer on the abdomen. No radiation is used during the procedure.
The FDA discourages any ultrasound for non-medical reasons.
Before the Procedure
Generally, no prior preparation, such as fasting or sedation, is required.
Notify your physician of all medications (prescription and over-the-counter) and herbal supplements that you are taking.
For an abdominal ultrasound, you may be asked to drink several glasses of water beforehand so you can arrive with a full bladder. A full bladder improves transabdominal imaging by reducing air in the pelvis. An empty bladder is best for transvaginal imaging.
Based on your medical condition, your physician may request other specific preparation.
During the Procedure
A fetal ultrasound may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician’s practices.
Generally, a fetal ultrasound follows this process:
- You will be asked to remove any clothing or other objects that may interfere with the scan.If asked to remove clothing, you will be given a gown to wear.
- You will lie on an examination table, either on your back or on your side.
- If a transabdominal procedure is being performed, a clear gel will be placed on the skin over the area to be examined.
- The transducer will be pressed against the skin (abdomen) and moved over the area being studied.
- If a transvaginal ultrasound is being performed, you will need to lie on your back in the same position used during a pelvic exam, with the legs bent and apart.
- The vaginal transducer probe will be covered with a clear sterile cover, and gel will be placed on the probe to ease the insertion of the probe.
- The technician will move the probe into different positions to capture necessary images.
- Once the procedure has been completed, the probe will be removed (if it was a transvaginal procedure), and you will be given tissue to wipe off excess gel (for either transvaginal or transabdominal procedure).