U

ULTRASOUND

Ultrasound is a method of assessing the fetus using
low-frequency sound waves to reflect off fetal tissue.
The ultrasound transducer produces ultrasound
waves, which bounce off tissue at different speeds de-
pending on its density. Most commercial ultrasound
equipment emits energy that is much lower than the
determined maximum safety standard. There are no
known reports of fetal damage from conventional di-
agnostic ultrasound.

There is no uniform agreement as to when ultra-
sound should be performed during pregnancy. Nev-
ertheless, ultrasound has become the predominant
method for determining fetal age, assessing fetal
anatomy, and monitoring fetal growth. The American
Institute of Ultrasound in Medicine recommends that
ultrasound be used in the first trimester to determine
fetal age, number, and viability (via visualization of
fetal heart activity). In the second and third trimes-
ters, the fetus can be scanned for anatomic abnormal-
ities, fetal growth, amniotic fluid volume, and
placental location.

See also: AMNIOCENTESIS; BIRTH; BIRTH DEFECTS;
PREGNANCY

Bibliography
Creasy, Robert K., and Robert Resnik. Maternal-Fetal Medicine.
Philadelphia: Saunders, 1999.
Gabbe, Steven, Jennifer R. Neibyl, and Joseph L. Simpson. Obstet-
rics: Normal and Problem Pregnancies. New York: Churchill Liv-
ingstone, 1997.
Garrett Lam

UMBILICAL CORD

The lifeline of the fetus during its stage of intrauter-

ine development, the umbilical cord averages 50 to

60 centimeters (20 to 23 inches) in length in a full

term pregnancy and connects the fetus to the placen-

ta. Contained within the cord are one umbilical vein,

which transfers from the placenta the oxygen and nu-

trients necessary for fetal growth and development,

and two umbilical arteries, which return the carbon

dioxide and metabolic waste products produced by

the fetus back to the placenta for elimination by the

mother. These blood vessels are wrapped within a

protective spongy material called Wharton’s jelly.

Umbilical cords are often coiled, an arrangement

that is thought to protect the blood vessels from the

external compressive forces of uterine contractions.

Some infants are born with the umbilical cord

wrapped around the neck or a body part. Rarely, fetal

movements can actually tie a knot in the cord. For the

most part, fetuses can tolerate these stresses well and

do not end up with major problems.

See also: BIRTH; PREGNANCY

Bibliography

Creasy, Robert K., and Robert Resnik. Maternal-Fetal Medicine.

Philadelphia: Saunders, 1999.

Gabbe, Steven, Jennifer R. Neibyl, and Joseph L. Simpson. Obstet-

rics: Normal and Problem Pregnancies. New York: Churchill Liv-

ingstone, 1997.

Garrett Lam

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