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New Ultrasound Protocol May Detect Birth Defects Non- Invasively
The fear of miscarriage makes many pregnant women aged 35 and older apprehensive about amniocentesis, the procedure used in the second trimester to detect chromosomal abnormalities in the growing fetus. Ultrasound scans that pose no danger to the fetus can, however, detect a majority of these and other anomalies earlier in pregnancy, substantially reducing the subsequent need for amniocentesis, according to physician researchers at New York University School of Medicine.
The Obstetrical and Gynecological Ultrasound Unit at NYU Hospitals Center is one of the leading centers in the United States exploring the development of less invasive, safer protocols for detecting fetal anomalies. The OB/GYN Department at NYU has the most sophisticated ultrasound equipment in the New York metropolitan area, including two three-dimensional sonogram machines, and is a testing site for the newest technology.
The unit also has developed a protocol of three ultrasound scans starting in the first trimester that will detect most abnormalities in the growing fetus. These efforts will help many pregnant women avoid invasive tests, which place the pregnancy at risk. Currently, only a handful of academic medical centers in the country have the expertise to perform and analyze accurately early ultrasound scans.
Ultrasound scans of pregnant women aged 35 and older can detect most abnormalities, says of Ilan Timor, M.D., Professor of Obstetrics and Gynecology at NYU School of Medicine, who pioneered the development of transvaginal ultrasound and is one of the world's leading experts in the technology.
"Early sonograms can provide reassurance to older pregnant women that their baby is healthy," says Dr. Timor, who also is Director of the Division of Obstetrical and gynecological Ultrasound. "The vast majority of all high-risk women can be assured that there aren't any problems and they may not need invasive testing. In only a small percentage of patients, early ultrasound will pick up markers for fetal anomalies, an indication for invasive testing with chorionic villus sampling (CVS) or amniocentesis."
Older women are at increased risk of giving birth to an infant with Down's syndrome or trisomy 21 (an extra copy of chromosome 21), a profound form of mental retardation. Traditionally, amniocentesis and CVS have been offered to women aged 35 and older to detect Down's syndrome and other genetic abnormalities; ultrasound, however, can significantly increase detection rates of these problems without increasing the number of invasive tests that are performed.
"There is a controversy in this country about the value of early ultrasound," says Michael Paidas, M.D., Assistant Professor in the Department of Obstetrics and Gynecology. "But we are finding that ultrasound can reliably detect most abnormalities with no risk to the fetus. Data suggests that 75% of women older than 35 years may elect not to undergo amniocentesis after first-trimester ultrasound screening."
Both amniocentesis and CVS are associated with a risk of miscarriage. Amniocentesis is an invasive procedure in which a needle is inserted into the fluid-filled sac surrounding the fetus. The fetal cells in the fluid are then analyzed for chromosomal defects. The risk of miscarriage is about 1 in 200 with this technique, which is usually performed around the 16th week of pregnancy. CVS uses either a catheter or needle to obtain cells from the placenta. It is performed earlier than amniocentesis at around 10 to 12 weeks of gestation The risk of miscarriage with CVS is about 1 in 100, but it may be less depending on the experience of the physician.
Transvaginal ultrasound provides extraordinarily detailed images of the early fetus by bouncing sound waves through the womb; structures of varying densities reflect the waves differently, which creates a picture that is viewed on a video monitor. Over the years, the technology has improved substantially and today, ultrasound machines enable early diagnosis for structural fetal anomalies.
"Women have many more options if anomalies are detected earlier," says Todd Rosen, M.D., Assistant Professor in the Department of Obstetrics and Gynecology at NYU School of Medicine, who specializes in prenatal diagnosis using ultrasound. "Certain conditions, when detected early, are even amenable to treatment in utero with medication or minimally invasive procedures."
In the Ultrasound Unit at NYU Hospitals Center, women at high risk for carrying a fetus with chromosomal abnormalities are offered three ultrasound scans. The first is performed with a transvaginal probe at 10 to 14 weeks of pregnancy. During this scan, an important anatomical feature called "nuchal translucency" (swelling in the neck region), can be discerned in the developing fetus that is an important marker for chromosome defects, including Down syndrome, and structural defects such as congenital heart disease. This marker has been the subject of several recently published studies in top medical journals, including The New England Journal of Medicine.
The second scan is performed transvaginally at 14 to 16 weeks of pregnancy. This detailed examination of the developing fetus's entire body, including the brain, can detect about 85% of structural abnormalities, says Dr. Timor. In addition, other markers for chromosome problems, including Down's syndrome, may be identified. This extremely informative and highly diagnostic transvaginal ultrasound is becoming important for early structural evaluation of the fetus. Fortunately, the vast majority of women are carrying healthy fetuses, and these mothers should be reassured by this sonogram.
A final transabdominal sonogram at 20 weeks confirms the results of the earlier scans and checks the fetus once more for any abnormalities. Altogether, this three-scan approach can provide a high degree of assurance that a fetus is developing normally without the need for amniocentesis. For example, a normal, first trimester ultrasound screen already halves the risk - a 35-year-old woman's plummets from 1 in 270 to 1 in 550. The risk may decrease even more with subsequent normal sonograms and serum marker screening.
Current research is focused on more precisely targeting all women, regardless of age, who are mostly likely at risk for giving birth to an infant with Down's syndrome.
Although women aged 35 and older are at greatest risk, women of all ages can give birth to an infant with the syndrome. In the last decade, blood tests at 15 to 16 weeks of pregnancy have increasingly been used to screen for Down's syndrome. When these tests are adjusted to the level that will identify about 60% of all Down's syndrome cases, only about 5% of women who test positive are at risk.
It isn't known which combination of blood tests and ultrasound screens will provide the most reliable way to detect Down's syndrome in the first trimester and substantially reduce the need for amniocentesis. The National Institutes of Health is now considering a study that will enroll over 50,000 women to address this question. NYU Hospitals Center will be one of the centers participating in the trial.
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