Procedure Overview

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). Fetal ultrasound provides a non-invasive way to evaluate the health of an unborn baby.

There are several types of fetal ultrasound, each with specific advantages 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:

  • head 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 an unborn 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.