One of the most frequent reasons for pregnant women to be referred for Maternal-Fetal Medicine (MFM) consultation is an ultrasound abnormality that may increase the risk of Down syndrome.
Down syndrome is a genetic abnormality involving an extra 21st chromosome. It is the most common chromosome abnormality to result in a live birth, occurring in about 1 in 1000 births. OB providers usually offer screening for Down syndrome to all pregnant women, in the form of blood work and screening ultrasound. When you have your screening ultrasound for fetal anatomy, there are some ultrasound findings that may increase the risk of Down syndrome.
Findings that are found more than half the time in Down syndrome fetuses, but more rarely in normal fetuses, are called major markers. These are:
- Thicker skinfold at the back of the neck
- Absent nasal bone
There are other ultrasound findings, called “minor markers,” that are found frequently in fetuses with Down syndrome but are also found in plenty of normal fetuses.
Let’s talk for a minute about these minor markers for Down syndrome since they seem to cause stress and anxiety due to their subtlety. One by one:
- Shorter arm and/or leg measurements: Down syndrome fetuses sometimes have femur or humerus bones that are somewhat shorter than expected. So when we see these measurements lagging, we wonder whether your baby may have an increased risk. But since babies come in all shapes and sizes, shorter bones may simply be a normal finding for your family. How tall are you? How tall is dad?
- Pelviectasis (extra fluid in the baby’s kidneys): Sometimes, we see a little enlargement or dilatation of the center of the kidney (the renal pelvis). This happens in up to 3-4% of normal fetuses but is more common in babies with Down syndrome. Mild dilatation typically resolves before the baby is born.
- Echogenic intracardiac focus (bright spot in the baby’s heart): A small bright spot is occasionally seen in one of the chambers of the fetal heart. This is called an echogenic focus and is a small mineral or calcium deposit on one of the cords that open and close the heart valves. An echogenic focus is found in 1-2% of all normal fetuses and is completely harmless. It is seen more frequently in babies with Down syndrome, but most babies with an echogenic focus are normal.
- Echogenic bowel (bright area in the baby’s intestines): Sometimes, there is an area of the baby’s intestine that appears brighter than the surrounding tissue. This is called the echogenic bowel. Like an echogenic intracardiac focus, the echogenic bowel itself is not harmful to the baby. It may have an unknown cause, or it may be caused by abnormalities like Down syndrome, cystic fibrosis (a genetic condition), swallowed blood (maybe you had some bleeding earlier in pregnancy, and the fetus swallowed this), or viral infections (especially CMV – cytomegalovirus). Further testing for these conditions will usually be offered if the echogenic bowel is seen.
- Single umbilical artery: Normally, a fetal umbilical cord has two arteries and one vein. These blood vessels carry blood between the baby and the placenta. In about 1% of babies, one of the umbilical arteries is missing. This finding is associated with an increased risk of Down syndrome. It may also be associated with other birth defects, like kidney and heart abnormalities. When we see a single umbilical artery instead of two, we look at the baby closely for any other abnormalities. We also watch fetal growth more carefully, as there may be a higher chance of the baby being smaller.
When an ultrasound marker for Down syndrome, major or minor, is seen on your ultrasound, you may be referred to the MFM specialist for genetic counseling and a more detailed ultrasound.
Generally speaking, minor markers occur in 1-2% of normal babies and more frequently in babies with Down syndrome. Minor markers are not the same as birth defects, and they may go away before the baby is born. If you are age 35 or more, or your baby has a major marker for Down syndrome or more than one minor marker, the Down syndrome risk may be higher.
The MFM specialist and/or genetic counselor will talk about risks and benefits of testing, and you can accept or decline any test that is offered. For minor markers, the discussion is more nuanced, because the Down syndrome risk may be less clear.
I talk more about the different tests for chromosome abnormalities in my post Testing for Chromosome Abnormalities.
When we talk through the particular set of findings in your baby, we try to determine how much your risk for Down syndrome is increased, then decide together what if any additional testing you would like in order to help settle this question.