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 depending 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 diagnostic ultrasound.

There is no uniform agreement as to when ultrasound should be performed during pregnancy. Nevertheless, 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 trimesters, the fetus can be scanned for anatomic abnormalities, 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. Obstetrics: Normal and Problem Pregnancies. New York: Churchill Livingstone, 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 placenta. Contained within the cord are one umbilical vein, which transfers from the placenta the oxygen and nutrients 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. Obstetrics: Normal and Problem Pregnancies. New York: Churchill Livingstone, 1997.

Garrett Lam

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