What to expect at your 18-to-20 week scan

Ultrasounds may be done at various times throughout pregnancy for different reasons. The mid-trimester ultrasound, also called the 18-to-20 week scan, examines your baby’s anatomy, growth and behavior. During this scan, doctors typically can determine the sex of the baby — if you want to know.

Additionally, the 18-to-20 week scan will:

  • Confirm pregnancy dates
  • Determine the number of fetuses
  • Examine the placenta
  • Assist in prenatal tests like an amniocentesis
  • Check the amount of amniotic fluid
  • Examine blood flow patterns
  • Measure the length of the cervix
  • How is an ultrasound scan done?

An ultrasound scan is a diagnostic technique that uses high-frequency sound waves to create an image of the internal organs. Pregnant women may have one of two types of ultrasounds.

In an abdominal ultrasound, gel is applied to the stomach and the ultrasound transducer glides over the gel on the stomach to create the image. A full bladder may be needed to perform an abdominal ultrasound in early pregnancy.

In a transvaginal ultrasound, a smaller ultrasound transducer is inserted into the vagina and rests against the back of the vagina to create an image. A transvaginal ultrasound produces a sharper image and is often used in early pregnancy.

What are the risks and benefits?

Fetal ultrasound has no known risks other than mild discomfort due to pressure from the transducer on the stomach or in the vagina. Radiation is not used during the procedure.

Transvaginal ultrasound requires covering the ultrasound transducer in a plastic or latex sheath. This may cause a reaction in women who are allergic to latex.

As with any test, results may not be 100 percent accurate. However, ultrasound can provide valuable information for parents and health care providers to care for both mother and baby. In addition, ultrasound gives parents a unique opportunity to see their baby before birth. This helps establish a bond early in the relationship.

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