An ultrasound test uses reflected sound waves to produce a picture
of organs and other structures inside the body. It does not use X-rays or other
types of possibly harmful radiation.
During an ultrasound test, a small handheld instrument called a
transducer (that emits and detects high-pitched sounds beyond the range of
human hearing) is passed back and forth over the area of the body being
examined. A computer analyzes the sound waves that are reflected back from
structures inside the body. These reflected sound waves are converted into a
picture and displayed on a video monitor. The images produced by ultrasound are
called a sonogram, echogram, or scan. Pictures or videos of the ultrasound
images may be saved for a permanent record.
Ultrasound is most useful for looking at organs and structures that
are either uniform and solid (like the liver) or fluid-filled (like the
gallbladder). Mineralized structures (like bones) or air-filled organs (like
the lungs) do not show up well on a sonogram. Since the uterus is filled with
amniotic fluid, it shows up well on a sonogram.
Fetal ultrasound is used to view a
fetus and the
placenta. It can be done as early as the 5th week of
pregnancy. Fetal ultrasound can be done by putting the transducer in the
woman's vagina (transvaginal) or by moving the transducer across her abdomen
(transabdominal). Transvaginal ultrasound is generally done early in a
pregnancy to detect a suspected
ectopic pregnancyand, occasionally, late in pregnancy
to determine the location of the
placenta. After about the 11th week of pregnancy,
almost all ultrasound tests are done using the transabdominal method.
Another ultrasound technique called Doppler ultrasound (or duplex
scanning) uses reflected sound waves to estimate the speed and direction of
blood as it flows through a blood vessel. Doppler ultrasound may be used to to
evaluate blood flow to the placenta and fetus.
Fetal ultrasound is the safest way to obtain information about the
fetus, such as size and position. Sometimes (in the second or third
trimester) it can also determine the sex of the fetus
and detect some structural abnormalities. X-ray tests are not used on fetuses
because the radiation can harm them.
- Fetal ultrasound usually can confirm pregnancy
6½ weeks after the woman's last menstrual period.
- Fetal
ultrasound can be done to predict a woman's due date and estimate the age of
the fetus. If fetal ultrasound is done during the 7th to 14th week of
pregnancy, the age of the fetus can usually be determined to within 5 days.
From the 14th to the 26th week of pregnancy, ultrasound to determine the age is
usually accurate to within 7 to 10 days. Past the 26th week of pregnancy,
ultrasound becomes less accurate at determining the age of the fetus because of
differences in growth rates.
- Fetal growth rates are checked by
comparing the size of the fetus's head, limbs, and body with standard
tables.
- The sex of the fetus can often, but not always, be
determined by about the 20th week of pregnancy.
See an illustration of
fetal
ultrasound.
