Prenatal Ultrasound Imaging
Ultrasound imaging makes it possible to view and record real-time images within the uterus. Ultrasound uses high frequency radio waves, which are transmitted into the uterus. The “echoes” that bounce back are detected and processed to create images.
Nuchal Translucency Ultrasound
A nuchal translucency ultrasound is an early-monitoring screen done to assess a baby’s chance of having Down syndrome and other genetic concerns.
“Nuchal ” refers to the area at the nape of the neck. Nuchal translucency is the fluid in the clear space just under the skin which is visible between the 11th and 14th week. A nuchal translucency ultrasound measures the amount of this fluid because babies with abnormalities tend to have more fluid than normal. Up to 75% of babies with Down syndrome will have increased thickness of the nuchal translucency.
This test also provides an early evaluation of the baby’s anatomy, which can help predict the likelihood of structural birth defects, such as congenital heart problems. It also establishes reliable identification of chorionicity (whether multiple babies share a placenta or have separate placentas).
Nuchal translucency measurements are not conclusive, but they can help determine the chance of a chromosomal disorder. Having a blood test done in conjunction with this test increases its accuracy. Parents often use this information to help them decide whether to have more invasive diagnostic procedures done.
Detailed Fetal Anatomic Ultrasound
A detailed fetal anatomic ultrasound is an anatomy scan routinely offered during the second trimester of pregnancy, usually between 18 - 22 weeks. This ultrasound checks to see that the baby is developing normally and looks at:
- The size of the baby
- The baby's anatomy, including heart, brain, bones, limbs and internal organs
- The position of the baby, umbilical cord and placenta
- The amount of fluid around the baby
- Fetal gender (if desired)
Additional testing may then be needed to confirm the screening results.
Follow-up or Complete OB Ultrasound
A follow-up or complete OB ultrasound evaluates a baby’s growth and answer questions about complex anatomical abnormalities. It is tailored specifically for each patient, depending on what the individual situation involves.
Targeted or Limited Ultrasound
Sometimes it is necessary to look at a specific element, such as amniotic fluid, the placenta or a specific part of the baby. For these targeted evaluations, a targeted or limited ultrasound is performed. The most common use of this type of ultrasound is to evaluate amniotic fluid.
Amniotic fluid is the watery liquid that surrounds the baby in the uterus. It provides a protective cushion for the baby and allows normal growth and movement. The amount of amniotic fluid varies, but typically is about a quart at 36 to 37 weeks of pregnancy.
An Amniotic Fluid Index (AFI) ultrasound assesses the amount of amniotic fluid. Too much fluid is called polyhydramnios and too little is called oligohydramnios. Either of these extremes may be associated with other problems in pregnancy or birth defects.
Performed during the second or third trimester, an AFI ultrasound is often recommended when:
- The mother has pre-existing conditions, such as high blood pressure or diabetes
- The pregnancy involves twins or more babies
- There are problems with the placenta
- Symptoms of too much fluid are present – such as abdominal discomfort, back pain and extreme swelling in the feet and ankles
- Symptoms of too little fluid are present – such as leaking fluid
- The baby is smaller than expected
Transvaginal ultrasound is a specialized form of ultrasound that allows a better view of small structures in early pregnancy and a targeted view of the cervix in later pregnancy. The cervix can change in length during pregnancy. When it becomes shorter prematurely, the risk of premature delivery may increase. A transvaginal ultrasound makes it possible to evaluate the cervix from inside the vagina.
Transvaginal ultrasounds are done to:
- See more clearly in the first trimester
- Monitor preterm cervical shortening
- Evaluate the lower part of the placenta and see its relationship to the cervix
- Evaluate other placental abnormalities
During a transvaginal ultrasound, a thin camera is covered with a sterile non-latex sheath and inserted a few inches into the vagina. It sends out high frequency sound waves, which reflect back to the detectors in the camera and allows a high resolution picture to be generated.
Biophysical Profile (BPP)
A biophysical profile is a noninvasive test that is usually done during the 3rd trimester to evaluate the baby's well-being.
Typically done after 32 weeks of pregnancy, the BPP consists of a targeted ultrasound done in conjunction with a non-stress test. The ultrasound is used to measure the baby's movement, muscle tone, breathing and amniotic fluid levels within the womb. The non-stress test is a separate part of the evaluation and measures the baby's heart rate using an electronic fetal heart monitor. The mother will lie on her side as two sensors are strapped to her abdomen. One sensor tracks the baby's heart rate and the other measures her contractions. The information is recorded and the doctor is able to compare the baby's heart rate in relation to its movements and the contractions.
The baby is then given a score of 0 (abnormal) or 2 (normal) in each of the five categories:
- Muscle tone
- Amniotic fluid volume
- Heart rate
The biophysical profile may be repeated once or twice a week during the last few weeks of pregnancy. This allows the doctor to assess the baby's health and responsiveness and determine if the baby needs to be delivered sooner than planned or further evaluation is needed.
Doppler Flow Ultrasound (Color Duplex Doppler)
A Doppler flow ultrasound measures the flow of blood in a baby's body and can even evaluate flow to the uterus and placenta. This allows the doctor to evaluate the baby's health and ensure that it is getting enough oxygen and nutrients from the placenta.
The Doppler flow study is most often used during the 3rd trimester of pregnancy in high-risk situations, including:
- Low amniotic fluid levels
- Twins sharing a placenta
- A baby that is small for gestational age
- Rh-incompatibility or other conditions that predispose to fetal anemia
Doppler flow ultrasound is noninvasive and is performed much like a regular ultrasound, using the same equipment. A specific blood vessel will be located and the Doppler function on the machine will evaluate it. The screen will display red and blue highlights, indicating the direction the blood is flowing.
If the doctor finds that the blood is not flowing as efficiently as it should and that the baby is not getting enough oxygen and nutrients to grow properly, the baby may need to be delivered earlier than planned.
A 3-D ultrasound can be used to evaluate a suspected abnormality. It provides a clear, still image of a baby in three dimensions. It is usually done between 26-30 weeks in order to get the best images. The procedure is conducted much like a regular ultrasound, except a special transducer and software are used.
To get a 3-D image, sound waves from the transducer produce echoes which are processed by a special computer program. The resulting 3-D images give the doctor an additional tool to evaluate the baby in order to confirm a diagnosis.
For most women, a 3-D ultrasound is unnecessary for diagnostic purposes, but allows the parents-to-be an intimate look at their developing baby. However, in some instances, a 3-D ultrasound may be medically necessary if an abnormality is suspected after a more standard ultrasound.
A fetal echocardiogram is a highly detailed ultrasound done to evaluate the structure and function of the baby’s heart. Often done between the 20th and 24th week of pregnancy, a fetal echocardiogram is typically recommended when:
- A routine ultrasound indicates a potential abnormality with the heart
- Genetic testing has revealed an abnormality
- One of the baby’s parents or siblings has a congenital heart defect
- The mother already had a condition before pregnancy, such as diabetes or lupus
- The pregnancy was conceived by IVF
Fetal echocardiogram is noninvasive and is performed much like a regular ultrasound, using the same equipment. A transducer is placed on the mother’s abdomen, where it sends out ultrasonic sound waves. The ultrasonic waves create a moving picture of the various parts of the heart, and show how blood moves through it. The screen will display red and blue highlights, which indicate the direction the blood is flowing. This ultrasound can take 45 minutes to two hours, depending on the complexity of the situation. It is painless and poses no risk to the baby.