First trimester US markers

The ultrasound screening for fetal chromosomal abnormalities is performed in the course of each fetal anatomy scan. A number of soft ultrasound markers is evaluated during the first trimester scan. Presence or absence those markers practically remodels the assessment of risk for an associated genetic abnormality.

Conversely, the absence of visible ultrasound markers or structural fetal abnormalities defines the woman as "low-risk" and usually no further investigations are recommended in such cases.

The fetal nuchal translucency (NT) is an ultrasound finding (a soft marker) and not a structural abnormality.

This is the area behind the fetal neck that accumulates some small amount of fluid in early gestation.

Usually, it can be visualized in the first and early second trimester as an hypoechogenic thin line in the nuchal area. In advanced gestation the nuchal translucency generally transforms into a skin fold - the nuchal foldness.

The nuchal translucency is one of the most important soft markers for chromosomal abnormalities, genetic syndromes and other structural abnormalities.

There is a normal range of the NT measurement in each gestational age. The NT thickness gradually increases throughout gestation. If the measurement is above the upper limit for the gestational age, the risk for chromosomal or genetic syndrome is increased. Furthermore, the increased nuchal translucency can be a result of congenital heart defect or another structural or skeletal abnormality. In cases where the fetus is completely normal it can resolve within a few weeks after the initial scan and can be associated with a normal perinatal outcome.

The nuchal translucency measurement is performed during the first trimester anatomy scan in all cases. The measurement can be taken transabdominally or transvaginally.

The nasal bone is another important soft marker for chromosomal abnormalities in the first trimester.

It is mainly used to assess the risk for trisomy 21 (Down syndrome).

In some cases with Down syndrome the nasal bone is absent (cannot be visualised completely) or it is much smaller than normal (hypoplastic).

However, such sonographic finding does not universally mean that the fetus has trisomy 21. Each case should be evaluated individually in the context of all other ultrasound findings, patient history, maternal age, biochemical screening, etc.

The nasal bone assessment is performed during the fetal anatomy evaluation between 11-13+6 w.g. In some cases, especially in early gestation, it might be difficult to perform the examination transabdominally and this may require a transvaginal scan.

Sometimes, in early gestation (before 12 w.g.) it is difficult to visualise the nasal bone due to the lack of calcification. As the pregnancy advances its reassessment may show a completely normal development in later gestation.

The assessment of blood flow in the tricuspid valve is another important tool for ultrasound screening for congenital heart defects and chromosomal abnormalities in the first trimester.

The tricuspid valve connects the right atrium to the right ventricle.

Normally, the blood flow across the valve is anterograde - with forward direction from the atrium to the ventricle. Thus, there is no backward blood flow when the valve is closed.

A backward flow from the right ventricle to the right atrium during systole is called tricuspid regurgitation.

Tricuspid regurgitation in the first trimester increases the risk for fetal cardiac abnormality. Since 40 to 50% of fetuses wtih Down syndrome have a cardiac defect, this sonographic finding (soft marker) is used in the combined assessment of risk for trisomy 21.

Tricuspid regurgitation does not universally mean that the fetus has a congenital heart defect or Down syndrome. Each case should be evaluated individually in the context of all other ultrasound findings, maternal age, biochemical screening, etc.

Tricuspid regurgitation can also be observed in structurally and genetically normal fetuses. In those cases the finding resolves spontaneously within a few weeks after the initial scan and eventually no adverse perinatal outcome is observed.

Whenever tricuspid regurgitation is diagnosed a further detailed ultrasound examination of the fetal heart (fetal echocardiography) should be carried out in later gestation.

The assessment of flow in the tricuspid valve is performed during the first trimester scan in all cases. It can be examined both transabdominally and transvaginally.

The assessment of flow in the Ductus venosus is another tool for assessment of risk for chromosomal and genetic syndromes, as well as for congenital heart defects in the first trimester.

Ductus venosus is a small vessel, present only in prenatal life.

It carries oxygenated blood from the umbilical vein through to the inferior vena cava into the fetal heart.

Ductus venosus regresses spontaneously shortly after birth.

The blood flow in the Ductus venosus, as well as in the tricuspid valve, is normally anterograde.

If it becomes retrograde during atrial contraction then a reversed a-wave is observed. This ultrasound finding increases the risk for chromosomal abnormalities (mainly Down syndrome) or congenital heart defects.

However, reversed a-wave in the Ductus venosus does not necessarily mean that the fetus is structurally or genetically abnormal. In 80% of cases with absence of other anomalies or soft markers for chromosomal abnormalities (isolated reversed a-wave), the prognosis is favorable and the pregnancy outcome is completely normal.

Ductus venosus assessment of flow is performed during the first trimester anatomy scan in all cases. It can be examined transabdominally or transvaginally.

The right subclavian artery originates from the first vessel coming from the aorta - the brachiocephalic trunk. Normally, it runs in front of the trachea and goes in the direction of the right arm.

An aberrant right subclavian artery (ARSA) is observed in about 2% of the normal population. In these cases the course of the artery is below and behind the trachea.

The aberrant right subclavian artery (ARSA) is one of the most important sonographic markers for chromosomal fetal abnormalities.

ARSA is seen in about 20% of Down syndrome cases.

Until recently, this ultrasound marker was used in prenatal screening for chromosomal abnormalities, mainly in the second trimester of pregnancy.

With advances of technology, improvement in quality of the ultrasound equipment and use of high-frequency transabdominal and transvaginal transducers, the demonstration of ARSA has become feasible in the first trimester.

The presence of aberrant right subclavian artery (ARSA) significantly increases the sensitivity of the combined biochemical screening for Down syndrome. For this reason, the study is an integral part of the examination of early fetal morphology.

The presence of ARSA does not necessarily mean that the fetus is affected by Down syndrome, it simply increases the risk. Therefore, this sonographic finding should be interpreted in the context of each specific case.

In most cases with ARSA, especially in the absence of other sonographic markers, the probability that the fetus is completely healthy is high.

Evaluation of the right subclavian artery can be performed by transabdominal and transvaginal ultrasound.

We offer to all our patients at MC MARKOVS the ultrasound assessment of all sonographic soft markers during the first trimester fetal anatomy scan. All examinations are performed by highly qualified healthcare professionals in a comfortable environment on modern ultrasound equipment.

For more information regarding the possibilities of having a comprehensive fetal anatomy assessment at MC MARKOVS, please contact your doctor or call at the registration desk.

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