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Perinatology

Perinatology

Ultrasound

What is ultrasound?

An ultrasound, also known as a sonogram, uses sound waves to create an image of an unborn baby. The sound waves, produced by an instrument called a probe, enter through the mother and bounce off of different parts of the baby at different speeds. For example, the sound waves return more quickly when hitting bone than fluid, so bone appears to be white, and fluid tends to be black on an ultrasound picture. Ultrasounds are performed by perinatologists and registered diagnostic medical sonographers.

When can an ultrasound be performed?

Ultrasounds can be performed at different stages in a pregnancy. A first trimester sonogram is the best tool for dating a pregnancy. However, little of the baby's anatomy can be visualized at an early gestational age. A crown-rump length, which is a measurement from the top of the baby's head to the bottom of the tail bone, is the best way to estimate the due date for the baby. A first trimester ultrasound can either be performed by inserting a probe into the vagina or by placing the probe on a woman's abdomen. The type of sonogram depends on the position and size of the baby.

An ultrasound can also be performed during the second trimester, typically between 18 and 22 weeks. An ultrasound performed by Maternal Fetal Medicine at this time is also known as a high resolution ultrasound, a targeted ultrasound, a complete anatomic survey or a level II sonogram. It is during this later scan that much of the baby's anatomy can be visualized. It is also possible to evaluate the placenta, umbilical cord, and amniotic fluid level. Many types of birth defects may be detected by a high resolution ultrasound, but not all problems can be seen.

Ultrasounds performed during the third trimester are generally used to monitor the growth and positioning of a baby. Ultrasounds are also performed during diagnostic procedures, such as amniocentesis, chorionic villi sampling, or percutaneous umbilical blood sampling. This allows the physician to visualize the baby and the needle used during the procedure, thus increasing the safety of the test.

What is a nuchal translucency measurement?

During ultrasounds performed between 10 weeks + four days and 13 weeks + three days, it is possible to measure the fluid that collects behind a baby's neck. This is known as a nuchal translucency measurement. Babies with excess fluid, or an increased nuchal translucency measurement, may have an increased chance of having a chromosome problem (such as Down syndrome or trisomy 18). It is important to remember that a nuchal translucency measurement is only a screening tool. Therefore, an increased or abnormal measurement may also be seen in babies who are healthy.

What is a fetal echocardiogram?

A fetal echocardiogram, which is generally performed between 20 and 22 weeks, takes a detailed view of the baby's heart through the use of ultrasound. It can view the four chambers of the heart, as well as the path of blood into, within, and out of the heart. Fetal echocardiograms may detect serious heart defects, but some heart defects cannot be seen prenatally.

What is a Doppler study?

Doppler is a technique that can help to assess blood flow. It is often useful in the evaluation of babies with intra-uterine growth restriction or IUGR. IUGR refers to an unborn baby that is not growing as expected.

What is a Biophysical Profile (BPP)?

A Biophysical Profile or BPP is a type of ultrasound that is done during the third trimester to check the well being of an unborn baby. The BPP assigns a score in the following four categories:

  • Fetal movement
  • Fetal Breathing
  • Fetal tone
  • Amniotic Fluid Level

It is generally used when a non-stress test reveals abnormal readings, or when the age of the pregnancy is too early to allow a NST to be performed.

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