Prenatal ultrasound

Definition

Prenatal ultrasound is a procedure performed during pregnancy to obtain images of the fetus.

Description

A prenatal ultrasound, also known as a sonogram, is a procedure in which a tool called a transducer is placed on a woman's abdomen so that images of the fetus in the women's uterus can be viewed on a monitor. The most commonly used ultrasound produces a two-dimensional image of the fetus, although the technology exists to create a three-dimensional image as well. Electrical energy coming in to the transducer is converted to high-frequency sound waves. The sound waves reflect off of any structures they contact and return to the transducer and are converted back to electrical energy. Generally, more dense structures, like fetal bones, are seen as bright white images on the screen. Less dense tissue, like organs and fluid, shows darker on the screen. Gel placed on the woman's abdomen acts as a medium and allows for more rapid transmission of the sound waves and a better image.

There are many reasons an obstetrician may recommend an ultrasound for a pregnant woman. Some of these reasons include getting an accurate gestational age (dating) of the pregnancy, determining viability of a fetus (i.e., heartbeat), determining if any structural changes are present in the fetus, and determining if there are problems in the fetal environment. A prenatal ultrasound is performed by a health care professional, including an obstetrician, a radiologist, and a maternal-fetal medicine specialist. The American Institute of Ultrasound Medicine recommends that physicians performing ultrasound complete an approved residency program, a fellowship, or postgraduate training that includes three months and 500 supervised ultrasound procedures. An ultrasound technician often performs the initial measurements and, if needed, the physician reviews the procedure and obtains additional images. Ultrasounds are sometimes referred to as either basic (level 1) or comprehensive (level 2 or 3), depending on the quality of the equipment used, the amount and detail of the fetal anatomy studied, and the training of the person performing the procedure.

Most pregnant women in the United States have one prenatal ultrasound. There is some debate about when this ultrasound should take place. Early ultrasounds, in the first trimester of pregnancy, are the most accurate way to determine how far advanced a pregnancy is and to set an accurate due date. However, these early ultrasounds do not allow for a detailed study of the fetal anatomy for any changes in development. An anatomical study of the fetus is best performed at about 18–20 weeks gestation. Women who are at increased risk for birth defects, genetic conditions, or pregnancy complications may have multiple ultrasounds. Studies have shown, and the American Institute of Ultrasound Medicine Bioeffects Committee has concluded, that there is no association between prenatal ultrasounds using sound waves and an increased chance for birth defects or other pregnancy complications. Studies have also shown that there is significant psychological benefit to parents from having ultrasounds performed. Prenatal ultrasounds provide parents with reassurance that the fetus is developing normally, or identify problems that can be investigated further.

First-trimester ultrasounds

The first trimester of pregnancy is considered to be the first 12 weeks of pregnancy, or the first three months. Prenatal ultrasounds in the first trimester may be performed either on the abdomen or vaginally; however, to get the clearest images in the first trimester, a vaginal ultrasound is performed. In this type of ultrasound, a smaller transducer is placed into the vagina, up to the cervix. By having the transducer closer to the uterus, the fetus, which is very small in the first trimester, can be more clearly viewed.

First-trimester ultrasounds are often performed for women, unsure of the dating of their last menstrual period, want to determine the correct gestational age. Early ultrasounds are best for dating because, early in a pregnancy, all fetuses grow at about the same rate. Therefore, measurements correspond well to gestational age. However, later in pregnancy, fetuses show more characteristics of their own growth pattern, and correlation to gestational age is not as clear. In general, first-trimester ultrasound measurements of gestational age are accurate to within about five days. Second-trimester ultrasound measurements of growth are accurate to about 8–12 days. Pinpointing a correct gestational age aids in determining an accurate due date and, consequently, in scheduling the proper tests at the proper times. One test that is offered to all pregnant women is called a maternal serum multiple marker screening test. This test screens a pregnancy to determine if there is an increased chance for Down syndrome , open spine defects, and another chromosomal condition called trisomy 18 . This test can be performed between 15 and 20 weeks of pregnancy, and may result in a false/negative or false/positive result if the due date of the pregnancy is not accurate.

First-trimester ultrasounds can also be used to detect multiple pregnancies, ectopic pregnancies (pregnancies located outside of the uterus), and the location of the placenta. The placenta is the organ between the mother and fetus that allows for the crossing of nutrients and oxygenated blood. Sometimes the placenta is located over the cervix, blocking the path the fetus would normally take to exit the uterus. Complications of this condition, called placenta previa, can be avoided by performing a cesarean section rather than a vaginal delivery. Another common reason for a first-trimester ultrasound is vaginal bleeding. First-trimester ultrasounds may be able to detect a pregnancy loss or a pregnancy that is not progressing normally. In general, the fetus is too small in the first trimester to view the anatomy in detail. However, a few serious structural changes to the fetus can be identified in the first trimester. These include conditions like anencephaly (a condition in which the fetal brain and skull does not develop), cystic hygromas (large fluid-filled sacs around the fetal neck), and conjoined twins .

Nuchal translucency

First-trimester ultrasounds can also be used to screen a pregnancy for chromosomal conditions, including Down syndrome. People with Down syndrome have mild to moderate mental retardation, a characteristic facial appearance, an increased chance for heart defects and other structural changes, and short stature. By measuring the translucent, or fluid-filled, area at the back of the fetal neck, it can be determined if there is an increased chance for a chromosomal change. An increased nuchal translucency indicates an increased chance for a chromosomal change. This measurement, in combination with an analysis of two proteins in the mother's blood called beta-hCG (human chorionic gonadotropin) and pregnancy-associated plasma protein A (PAPPA) in the first trimester of pregnancy, is referred to as first-trimester screening for aneuploidy (changes in chromosome number).

By performing a first-trimester screening between 10 and 14 weeks of pregnancy, about 80% of fetuses with Down syndrome can be detected. Most fetuses with two other chromosomal conditions, trisomy 13 and 18, can also be detected by first-trimester screenings. Therefore, first-trimester screening for aneuploidy is considered as effective as the more traditional screening for aneuploidy performed in the second trimester. However, The American College of Obstetricians and Gynecologist recommended that individuals who perform prenatal ultrasounds and take measurements of the nuchal translucency complete additional training to ensure that they are taking this measurement correctly. The recommendation also stated that there should be ongoing monitoring of the accuracy of this measurement, that appropriate counseling about screening and testing options for aneuploidy should be provided to women wishing to have first-trimester screening, and that the screening should only be performed when further diagnostic testing indicates an increased risk for aneuploidy. The diagnostic test for chromosomal changes available in the first trimester of pregnancy is called a chorionic villus sampling (CVS). This test can be performed between 10 and 12 weeks of gestation. However, CVS is not as commonly available as amniocentesis , the second-trimester test for chromosomal changes.

Screening for aneuploidy in the first trimester is a more recent event and allows women to have diagnostic testing earlier and to make decisions about continuing or discontinuing a pregnancy earlier. Ending a pregnancy in the first trimester or early second trimester is safer for the mother than later pregnancy terminations. In fetuses with an increased nuchal translucency and normal chromosomes, follow-up ultrasounds are recommended as heart defects, other structural changes in development, and other genetic syndromes can still be present.

Second-trimester ultrasound

Ultrasounds in the second trimester of pregnancy are most often performed using the transducer on the maternal abdomen. Vaginal ultrasound may be used in situations where there is a problem with good visualization of the fetus, such as in obese women. In the second trimester, the fetus is larger and it is possible to see more detail of the development of the fetal anatomy. Anatomy scans of the fetus are most often performed at 18–20 weeks of gestation and can also include images of the placenta, amniotic fluid, umbilical cord, and the sac containing the fetus.

It is also important to look for fetal movement. The absence of movement can indicate the presence of a condition affecting the muscles, bones, or central nervous system. There are many other fetal anomalies that can be detected as well. The finding of a structural anomaly often increases the chance for aneuploidy. Patients may be offered additional tests for more information. When one fetal anomaly is noted on ultrasound, it is important to study the fetus carefully for the presence of other anomalies, since anomalies often occur simultaneously. The prognosis for a fetus when an anomaly is found depends on the severity of the anomaly, the presence of other anomalies, and the presence of a chromosomal condition. Fetuses with mild isolated anomalies have the best prognosis.

Fetal anomalies

A detailed second-trimester ultrasound allows for the identification of structural changes in development which, in themselves, may be a significant concern, or may be a sign that additional problems like chromosomal changes are present. About 2–3% of all live born infants have a birth defect.

A detailed ultrasound studies many structures in the fetal head and brain. There is a measurement of the amount of fluid in the ventricles, or areas, of cerebrospinal fluid in the brain. Too much fluid in the ventricles of the brain is a condition called hydrocephalus . Depending on the degree of hydrocephalus, there can be underdevelopment of the brain and severe mental retardation. The presence of the two main lobes of the brain and the skull are noted to rule out a type of open spine defect called anencephaly. The lobes of the brain are studied to detect midline defects of the brain such as holoprosencephaly , in which the lobes of the brain have not divided properly. The cerebellum, an organ at the back of the brain, is studied because changes in the shape of the cerebellum or the bones in the front of the skull are indicative of an open spine defect, or spina bifida . The size of the head is measured, because a small head size, called microcephaly, can result in mental retardation. The face is studied for the presence of a cleft lip, a small jaw, or either widely or narrowly spaced eyes.

A study of the fetal chest and abdomen will determine if the organs are located on the normal side, or if they are reversed. Ultrasound can detect the presence of diaphragmatic hernia, in which there is a hole in the diaphragm and the intestines are herniated through it into the chest. Abdominal wall defects, in which there is an opening in the abdomen and the intestines are located outside the body, can be detected. Changes to the kidneys, such as cystic, or missing, kidneys and cystic malformations of the lungs, can be detected. The heart is a particularly complex organ and can be difficult to study with ultrasound. The more severe heart defects can be identified with ultrasound, but many smaller structural changes to the heart are missed with ultrasound. When a heart defect is suspected, a more specialized ultrasound of the fetal heart, called an echocardiogram, can be performed. Ultrasound evaluation should also include views of the fetal bladder and stomach. The sex of the fetus can often be seen after 18 weeks of gestation.

A careful study of the fetal spine can detect the presence of spina bifida. Changes in the bones of the arms and legs can also be noted. In particular, an inward curving of the feet or club feet can be seen as well as missing limbs. Shortened or curved long bones are a sign of a skeletal dysplasia . There are many types of skeletal dysplasias with varying degrees of severity. Some skeletal dysplasias also include broken bones, short ribs, or changes to the bones in the spine.

Not all changes in development can be detected by ultrasound. The detection rate for individual anomalies with ultrasound varies, and centers that perform ultrasound should collect their own data before providing statistical detection rates to patients. In addition, there can be instances when a condition is diagnosed by ultrasound, but not truly present, or when a condition is misdiagnosed. Patients should be informed of the limitations of ultrasound before the procedure is performed.

Ultrasound markers

With the advancement of ultrasound technology, additional findings on ultrasound have been identified. These findings are not anomalies, or structural changes, that in themselves cause any problem in development. However, these findings have been associated with an increased chance for chromosomal changes in a fetus. Therefore, these findings have been referred to as markers, or soft signs, of aneuploidy. For instance, fluid-filled cysts called choroids plexus cysts have been seen in fetuses with aneuploidy. However, the finding of a marker alone in a woman with no other increased risk for aneuploidy is not likely to be significant. Finding multiple markers does increase the chance for aneuploidy. When a marker is seen in the fetus of a woman at increased risk for aneuploidy based on advanced maternal age, maternal serum multiple marker screening, or family history, the patient should receive additional genetic counseling to discuss the finding and how it affects the risk for aneuploidy and the additional testing options. When not associated with aneuploidy, these markers are most often of no significance to the health of the fetus.

Fetuses with several chromosomal conditions can survive to full term. Those conditions include trisomy 21 (Down syndrome), trisomy 18, trisomy 13, and Turner syndrome (45,X).

About half of children with Down syndrome have a heart defect, some of which can be detected by ultrasound. In addition, fetuses with Down syndrome may have various gastrointestinal changes or a cystic hygroma (fluid-filled cyst around the neck) that may be detected by ultrasound. Ultrasound markers for Down syndrome include inward curving of the fifth finger, an increase of fluid in the kidney, short long bones in the upper arm or leg, extra skin at the back of the neck, a bright spot in either the heart or bowel, absence of the nasal bone, and fluid-filled cysts in the brain. When any of these findings is seen in a woman with an increased chance for Down syndrome, her risks may be increased. If an ultrasound is performed in a woman with an increased risk for Down syndrome, and none of these findings is seen, her risks may be lower than previously estimated. However, risk reduction based on ultrasound is controversial and should not be done unless an ultrasound center has collected their own data on detection of Down syndrome with ultrasound.

Trisomy 18 is a rare, severe condition caused by the presence of an extra chromosome 18. Babies with trisomy 18 have severe neurological problems and mental retardation and usually do not survive more than a few months. The most common ultrasound findings in fetuses with trisomy 18 include heart defects and skeletal changes. Fetuses with trisomy 18 have a characteristic hand position that can be noted, clubbed feet, an unusual shape to the feet called rocker-bottom feet, and an unusual shape to the skull called a strawberry-shaped skull. Many other anomalies of other organs have also been noted in fetuses with trisomy 18, including diaphragmatic hernia, open spine defects, and significant growth delay. Markers for trisomy 18 include a thickened nuchal fold, extra fluid in the kidneys, choroids plexus cysts, and a single umbilical artery. Most, but not all, fetuses with trisomy 18 will have some feature of the condition identified on ultrasound.

Trisomy 13 is also a rare, severe condition caused by the presence of an extra chromosome 13 and associated with multiple birth defects, mental retardation, and a shortened lifespan. Ultrasound findings in fetuses with trisomy 13 include heart defects, brain malformations like holoprosencephaly, small head size, cleft lip, kidney malformations, extra fingers or toes, and growth delay. Markers for trisomy 13 include an increased nuchal fold and a bright spot in the heart. Most, but not all, fetuses with Trisomy 13 will have a feature of the condition noted on ultrasound.

Turner syndrome is a condition in which a female is missing the second X chromosome. Females born with Turner syndrome have an increased chance for heart defects and kidney problems. Girls with Turner syndrome have short stature, and will experience infertility as adults. Many pregnancies of fetuses with Turner syndrome miscarry. Ultrasound appearance of these pregnancies often includes a large cystic hygroma, and generalized fluid accumulation throughout the fetal body, called hydrops or edema. Fetuses with Turner syndrome that survive to delivery will sometimes show heart defects, kidney problems, or a cystic hygroma on an ultrasound.

Another way to study the fetal well-being is to study the fetal environment and associated pregnancy structures. If a fetus has oligohydramnios (a low amount of amniotic fluid) in the sac surrounding it, the lungs may not develop and the fetus may not survive. Oligohydramnios may occur because of absence of the kidneys, other structural defects, or due to a rupturing of the membranes or sac surrounding the fetus. Polyhydramnios (an excess of amniotic fluid) may also be a sign of structural changes, especially blockages in the urinary tract system, and can lead to premature delivery of a fetus. There can also be problems when the inner membrane of the sac that holds the fetus tears. The torn strands of amnion can wrap around the fetus and cut off blood flow to those areas. The presence of amniotic bands can disrupt the formation of limbs or any structure they wrap around and cause deformations that range from mild to lethal. There can be problems with the umbilical cord if the cord is too long or too short, has knots in it, is entangled around the fetus, has cysts in it, or is not inserted into the center of the placenta. The placenta may also cause or be associated with problems if it is too small, too large, split, located over the cervical opening, detaches from the uterus, or has tumors in it. Molar pregnancies occur when the placenta is swollen and cystic and no fetus is present. In a partial molar pregnancy, a small, malformed fetus with a chromosomal abnormality called triploidy is often present. In triploidy, a fetus has a whole extra set of chromosomes, so that the total number is 69 instead of the usual 46.

Third-trimester ultrasound

Ultrasounds in the third trimester, or last three months, of the pregnancy are often done to monitor fetal growth. If a fetus is macrosomic (large), as is often the case in maternal diabetes , then the fetus is monitored for timing of delivery. Some fetal anomalies that are present earlier do not show features until the third trimester, and some fetal anomalies, like hydrocephalus, may not develop until the third trimester. Ultrasound may be used in the third trimester for fetuses with known anomalies to monitor progression and consider delivery techniques and timing.

Another important use of ultrasound is to guide prenatal procedures such as chorionic villus sampling (CVS) and amniocentesis. In CVS, a physician inserts a catheter into the vagina, through the cervix, and into the placental tissue using ultrasound guidance. By suctioning a small sample of the placental tissue, a sample can be obtained for chromosomal studies. CVS is performed at 10–12 weeks gestation. Amniocentesis is a test performed at 15–20 weeks gestation to obtain a sample of amniotic fluid from the sac of fluid around the fetus. This fluid contains fetal cells that can be used for chromosomal analysis of the fetus. During amniocentesis, a pocket of amniotic fluid located away from the fetus is targeted. Ultrasound is used to identify a pocket of fluid and used to guide the insertion of the needle into the targeted area.

Ultrasound is useful tool in the care of pregnant women. There are many uses for prenatal ultrasound and no identified risk with the procedure.

Resources

BOOKS

Chervenak, F., and S. Gabbe. "Obstetric Ultrasound: Assessment of Fetal Growth and Anatomy." In Obstetrics: Normal and Problem Pregnancies, Fourth Edition, edited by Gabbe, S., J. Niebyl, J. L. Simpson. Philadelphia: Churchill Livingston, 2002.

Ville, Y., K. Nicolaides, and S. Campbell. "Prenatal Diagnosis of Fetal Malformations by Ultrasound." In Genetic Disorders and the Fetus: Diagnosis, Prevention and Treatment, Fifth Edition, edited by Aubrey Milunski. Baltimore, MD: The Johns Hopkins University Press, 2004.

PERIODICALS

American College of Obstetrics and Gynecology, Committee on Obstetric Practice and Committee on Genetics, Committee Opinion No. 296. "First-trimester Screening for Fetal Aneuploidy." Obstet Gynecol 104 (2004): 215–217.

Brigatti, K. W., and F. D. Malone. "First-trimester Screening for Aneuploidy." Obstet Gynecol Clin N Am 31 (2004): 1–20.

Goldberg, James. "Routine Screening for Fetal Anomalies: Expectations." Obstet Gynecol Clin N Am 31 (2004): 35–50.

Lazarus, Elizabeth. "What's New in First-trimester Ultrasound." Radiol Clin N Am 41 (2003): 663–679.

Marino, Teresa. "Ultrasound Abnormalities of the Amniotic Fluid, Membranes, Umbilical Cord, and Placenta." Obstet Gynecol Clin N Am 31 (2004): 177–200.

Nicolaides, Kypros. "Nuchal Translucency and Other First-trimester Sonographic Markers of Chromosomal Abnormalities." American Journal of Obstetrics and Gynecology 191 (2004): 45–67.

Stewart, Teresa. "Screening for Aneuploidy: The Genetic Sonogram." Obstet Gynecol Clin N Am 31 (2004): 21–33.

Sonja R. Eubanks, MS, CGC