What Is a Prenatal Ultrasound?
A prenatal ultrasound is a safe and painless test that uses sound waves to make
images that show the baby's shape and position. It can be done in the first, second,
or third trimester, depending on what the health care provider is looking for.
The American College of Obstetrics and Gynecologists (ACOG) recommends at least
one ultrasound during a pregnancy, but many women have two or more. Women with high-risk
pregnancies might have multiple ultrasounds during their pregnancy.
Why Are Prenatal Ultrasounds Done?
Ultrasounds were once used only in high-risk
pregnancies, but have become so common that they're often part of routine prenatal
care.
During an ultrasound, sound waves are bounced off the baby's bones and tissues
to make an image showing the baby's shape and position in the uterus. Also called
a sonogram, sonograph, echogram, or ultrasonogram, an ultrasound is done to:
- confirm the expected date of delivery
- detect pregnancies outside the uterus
- see whether there might be more
than one fetus
- see whether the fetus is growing at a normal rate
- record fetal heartbeat or breathing movements
- check the amount of amniotic fluid in the uterus
- check the position of the placenta (which can sometimes block the baby's way out
of the uterus) in late pregnancy
- guide doctors during other tests, like amniocentesis
- find structural defects that may indicate Down
syndrome, spina bifida,
or anencephaly
- find other problems, such as congenital heart defects,
cleft lip or palate, and gastrointestinal or kidney problems
What Happens During a Prenatal Ultrasound?
To prepare for a prenatal ultrasound, you might be asked to drink lots of fluids
or not pee for a while so that your bladder will be full for the exam.
You'll change into a cloth gown and lie on a table. The room is usually dark so
the images can be seen clearly on the computer screen. A technician (sonographer)
trained in ultrasound imaging will spread a clear gel on your abdomen. This gel helps
with the transmission of the sound waves.

The technician will move a small wand (a transducer) over the gel. The transducer
emits high-frequency sound waves and a computer measures how the sound waves bounce
back from the body. The computer changes those sound waves into images. Sometimes
a doctor will come in at the end of the test to meet with you and take a few more
pictures.
The prenatal ultrasound is painless. You may feel a slight pressure on the belly
as the transducer is moved over the body, and the gel may feel wet or cold. The test
usually takes less than 30 minutes.
Sometimes an ultrasound test can be done through the vagina (called a transvaginal
ultrasound) early in the pregnancy when the uterus and ovaries are better
seen from that angle.
Some ultrasounds can show the fetus in three dimensions, like a photograph (a 3D
ultrasound), and sometimes show movement (a 4D ultrasound).
Doctors might use these to look for birth
defects.