Prenatal ultrasound, also known as sonogram, is an imaging technique that is used to look at a fetus during the pregnancy. Ultrasound involves using sound waves that reflect off of the different parts of the baby to generate a picture. This picture can be used to take measurements, which can be used to verify how long a pregnancy is along and to monitor growth throughout the pregnancy. Ultrasound can also be used to screen for birth defects, such as spina bifida, heart defects, and cleft lip.

It is fairly common to have at least one ultrasound during pregnancy. However, some women may have an earlier ultrasound or more ultrasounds if they have bleeding, pain, or other concerns are identified on an initial ultrasound.

Currently, there is no reliable evidence that ultrasound is harmful to a developing baby. For example, there have been no associations between ultrasound and birth defects, childhood cancer, or developmental problems later on in life.

However, according to the American College of Obstetricians and Gynecologists patient information on ultrasound, it is possible that harmful effects could be identified down the road. Their recommendation is that ultrasounds be performed only for medical reasons by qualified health care providers.

There are generally two types of ultrasounds that are used during pregnancy to screen for genetic conditions and birth defects: the first trimester ultrasound, and the anatomy ultrasound.


First Trimester Ultrasound

The first trimester ultrasound is usually done in the first trimester, and the baby is small at this time, only a few inches in length. Although the fetus is still very little, there are some important things that this ultrasound can tell us:

  • It can verify the presence of a viable pregnancy
  • It can estimate how far along the pregnancy is
  • It can see if there is more than one fetus (twins, triplets, etc)
  • It measures a pocket of fluid behind the fetus’ neck called the nuchal translucency (NT)
    • If the NT measurement is larger than expected, it can increase the risk for chromosome conditions, such as Down syndrome, as well as heart defects

A normal first trimester ultrasound does not guarantee a healthy baby, but it does lower the chance for certain birth defects and genetic conditions.

Anatomy Ultrasound

The anatomy ultrasound, usually done around 20 weeks, is where a lot of measurements are taken. The fetus is checked out from head to toe, and measurements are taken of its head, limbs, and abdomen. During this screening test, the doctor will also look at important organs, such as the heart, the brain, and the kidneys. This is usually the ultrasound where the gender of the baby can be determined. The anatomy ultrasound can also make sure the placenta looks healthy, and that there is the right amount of amniotic fluid around the baby. Much like the first trimester ultrasound, a normal anatomy ultrasound does not guarantee a healthy baby, but it can lower the risk for certain birth defects and genetic conditions.

While most anatomy ultrasounds are completely normal, there are occasions when the doctor may find something that looks different on an ultrasound. These difference can be broken down into two categories: structural abnormalities and minor markers.

Structural abnormalities (birth defects)

Structural abnormalities, which we can also call birth defects, are relatively rare and incredibly variable. They can range from very minor and easily treated to severe and even life-threatening. Some examples of birth defects are:

  • Heart defects
  • Cleft lip and/or palate
  • Club foot
  • Spina bifida
  • Polydactyly (extra fingers or extra toes)

A birth defect can be an isolated finding, meaning that there are no other related health problems, or they could be an indicator of an underlying genetic condition or syndrome that may be associated with other health and developmental concerns.

Minor Markers

A minor marker is a difference that is seen on ultrasound that does not cause any health concerns itself, but results in an increased risk for certain genetic conditions. Most of the time, a minor marker is a slight variation in development that is not linked to an underlying health issue.

For example, shorter than average femur (thigh) bones are considered a minor marker that increases the chance for Down syndrome. This is because many babies with Down syndrome have shorter than average thigh bones. But there are also many babies who do NOT have Down syndrome but DO have shorter than average thigh bones. There are also some babies with Down syndrome that have average thigh bones. If a minor marker is found on your ultrasound, it is important to talk with your doctor and/or genetic counselor to understand what that marker means for your pregnancy.

What are some other minor markers that may be seen in the 2nd trimester?

  • Shortened or Absent Nasal Bone
  • Choroid Plexus Cyst
  • Echogenic Intracardiac Focus
  • Increased thickness of the nuchal fold
  • Renal Pyelectatsis/Hydronephrosis (enlarged kidneys)
  • Short femur bones
  • Short humerus bones
  • Single Umbilical Artery (SUA)

What if a minor marker or multiple markers are seen on my ultrasound?

You should be talk with a medical professional that can provide you more information about what these findings may mean for your baby. Depending on what marker(s) is seen, your doctor may offer diagnostic testing, such as amniocentesis to evaluate for certain genetic and chromosomal conditions like Down syndrome. It is always a personal choice whether to undergo any prenatal testing.

If you decide not to undergo amniocentesis it may be recommended that you have a follow-up specialized ultrasound to more carefully evaluate the baby’s heart, called a fetal echocardiogram. Babies with chromosome conditions including Down syndrome have a higher chance for a heart defect. If an ultrasound suggests an increased risk for a chromosome condition, it may be important for planning your delivery to determine if the baby is likely to have a heart defect.

What if a structural abnormality is detected?

Next steps will depend on the specific finding or concern. Sometimes an abnormally on ultrasound is associated with a higher likelihood of certain genetic or chromosomal condition in the baby and further diagnostic testing, such as amniocentesis, may be offered. While amniocentesis can provide a lot of information, there is a risk of miscarriage, so the decision about whether to undergo an amniocentesis is yours.

Regardless of whether or not you decide to do follow-up diagnostic testing and regardless of the results, it is not uncommon for more follow-up ultrasounds to take place after an ultrasound finding has been identified.  It may be recommended that you deliver your baby at a hospital with a critical care nursery that is equipped to address any special medical needs your baby may have. Some structural abnormalities can be repaired with surgery and some cannot.

In the rare situation that a baby is diagnosed with life threatening abnormalities on ultrasound, expectant parents should be counseled about their options. Some women choose to end their pregnancy if serious abnormalities are discovered. Some women decide to carry their pregnancies and may be supported through a perinatal hospice program if their baby is not expected to survive.

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Last updated on Apr 7th, 2018