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High Quality Ultrasound is Best Performed in All Three Trimesters

In the first trimester, we establish pregnancy due date, count the number of fetuses and whether multiples are fraternal or identical, look for major fetal problems, establish the location of the placenta and use measurement of the nuchal translucency and other structures to alter the a priori risk of chromosomal anomalies such as Down syndrome.

In the second trimester, we do detailed anatomical analysis of the fetus from head to toe. The "anatomy scan" is usually done at about 20 weeks so that there is enough development particularly of the heart and brain for detailed visualization of structures and function.

In the third trimester, we follow the rate of growth of the fetus, the proportion of fetal structures, amniotic fluid volume, and placental function. In addition to growth per se, we can perform biophysical profiles which are very good predictors of placental function and fetal health.

First-Trimester Ultrasound, Nuchal Translucency, and First-Trimester Instant Risk Assessment™ Screening

Ultrasound imaging looks at the fetus within the abdomen. First-trimester ultrasound is the most reliable way of establishing the "due date" of pregnancy; of determining the number of fetuses, and whether multiple fetuses are identical or fraternal; and of screening for elevated risk of genetic abnormalities, including Down syndrome and cardiac, neural, and limb abnormalities.

During ultrasound, the nuchal translucency (NT) can be established; this measurement determines the thickness of the back of the fetal neck. For over nearly 20 years in over 2,000,000 pregnancies NT and blood studies for free  hCG and PAPP-A have been shown to be a very reliable indicator of many genetic risks.

First-trimester screening results usually either increase or decrease the age-related risk of the pregnant woman for Trisomy 21 (Down syndrome), Trisomies 13 and 18. For example, the results may describe a 26-year-old woman as having the risk of carrying a fetus with Down syndrome equivalent to that of a 40-year-old (or vice versa). By performing first-trimester combined screening (free β hCG, PAPP-A, and nuchal translucency ultrasound) about 85% of pregnancies with Down syndrome can be identified in the first trimester. (The second trimester quadruple blood screen can only detect about 60% of cases.) Patients can then use this information to decide whether they wish to have definitive testing, such as CVS, to obtain a specific diagnosis in the first trimester.

First-trimester screening is more sensitive than that in the second trimester or than using maternal age alone. Also, early screening allows early diagnosis of fetal abnormalities, giving parents more time to exercise treatment and management options.

The goal of screening is to maximize the chance of finding a problem that might be there while at the same time minimizing the number of women who require diagnostic (definitive) testing to be certain. Patients have a finger stick blood sample taken at 9-11 weeks. Then, at 11-and-a-half to 13 weeks 6 days, we perform the first trimester ultrasound and measure the thickness of the back of the neck—referred to as the Nuchal Translucency (NT) measurement. Accurate measurement of the NT is critical, as its results are used in a formula combined with the gestational age of the fetus (as measured by its length), the age of the mother, and the results of the hCG and PAPP-A blood tests. While the patient is in our office having the US, we can instantly compute the adjusted risk. Low-risk patients can be reassured and higher risk patients can be offered diagnostic testing via CVS, which provides a definitive answer and can usually be performed at the same visit.

It is important for all patients to remember, however, that screening results do not give a definitive answer but merely alter odds. Whatever the Down syndrome risk may be on screening, the risk of other issues such as sex chromosome abnormalities, rings, translocations, and inversions (on the basis of a karyotype) can be made no lower than about 1/500. Now that microarrays are routinely available for our patients, that minimal number is actually at least 1 100 (which is comparable numerically to what we have been counseling 38 year olds). Therefore when we counsel patients, we believe that the lowest possible risk internalized by patients in their decision-making process should be 1/100 if the computed Down syndrome risk is reported as lower than that.

Second Trimester Ultrasound

Genetic ultrasound is part of every diagnostic procedure we perform. Assessment of fetal structure and sometimes function allows for the reassurance of normality in the majority of instances. Unfortunately, we also find pregnancies in which there are significant structural abnormalities that can vary from minor to severe to lethal. We use the latest machinery that provides the highest resolution in 2, 3, and 4 dimensions.

Four Chambers

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2nd Trimester Brain Structures

2nd Trimester Spine

2nd Trimester facial 3D

Third Trimester Ultrasound

Assessment of continued fetal growth is important to assuring fetal health. When appropriate, we can do further tests such as fetal biophysical profiles to document fetal well-being.

Evaluation of fetal growth is best performed in all three trimesters to assure continued, appropriate growth. Further evaluation of fetal health can be obtained by biophysical profiles what document amniotic fluid volume, fetal movement, breathing, and tone. A separate "non stress test" on monitoring is often added. Philosophically, we like "data" and are much more aggressive in getting hard data from ultrasounds than many programs, but we have been able to intervene in selected situations to get good outcomes that might not have been possible with a more laid back approach.


  • Quick Contact Info

    Dr. Mark I. Evans (MD PLLC)

    Phone: 212.288.1422
    Fax: 212.879.2606
    Email: Evans@CompreGen.com 131 E 65TH ST
    NEW YORK NY 10065