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Pregnancy Ultrasound: What to Expect

Reviewed by James Brann, M.D.

Pregnancy Ultrasound - Second Trimester

Learn what to expect at your prenatal ultrasound examination.

At some point between pregnancy week 18 and 22, you will have a second trimester ultrasound. By this stage in pregnancy, all of your baby's major organs, arms and legs, and other body parts have developed enough that they can be easily seen and examined with an ultrasound scan. Because of this fact, the second trimester ultrasound scan will be more in-depth than the one you had in the first trimester.

What Happens During a Pregnancy Ultrasound?

During your second trimester ultrasound, the ultrasound technician (called the sonographer) will measure your baby to ensure that he or she is growing normally.

After 20 weeks pregnant, your baby's growth is no longer measured from "crown to rump." Instead, the doctor or sonographer will determine your baby's size using a series of different measurements. These include the circumference of your baby's head, the circumference of your baby's tummy, and the length of the baby's upper leg bone (femur). Using these measurements, your doctor will be able to tell whether your baby falls within the normal range for babies at pregnancy week 20.

In addition to measuring your baby's size, the following areas of your baby's body will be examined:

  • The head and brain – The first area of your baby's body that will be examined is the head and brain. The sonographer will examine the hindbrain (also called the cerebellum) at the back of your little one's brain, as well as the fluid-filled areas of the brain. Any abnormalities may indicate that your child has a chromosomal defect, like Down syndrome.

  • The spine – Your little one's spine will be examined for any defects. The sonographer will look at the spine to ensure that your baby's vertebrae is properly aligned and that your infant's skin completely conceals the spine. Any abnormalities may mean that your child has spina bifida or a spinal birth defect.

  • Arms and legs – The bones in your developing baby's arms and legs will be looked to rule out any birth defects. If the bones in the arms or legs are shorter and not as long as they should be, this may be a marker that your infant has Down syndrome.

  • Diaphragm and stomach – To ensure that your little bundle of joy is growing healthy, the stomach, diaphragm, and abdominal wall will be examined rule out problems and birth defects.

  • Bladder and Kidneys – In order to rule out any blockage or defects in the bladder or kidneys, the sonographer will examine these organs. The sonographer also must double check that your baby has both kidneys.

  • Umbilical cord and amniotic fluid – The umbilical cord is also examined and the number of blood vessels is counted. The sonographer will also look at the level of amniotic fluid in the womb.

The position of your placenta will also be examined at your second trimester ultrasound scan. If you have a low-lying placenta, your healthcare provider will schedule follow-up ultrasound scans to rule out placenta previa – a pregnancy complication in which your placenta covers either partially or fully the cervix (the opening of your womb).

If you are diagnosed with a low-lying placenta, try not to worry about it at 20 weeks pregnant. In a majority of cases, a low-lying placenta will move away from the cervix the further along you are.

For most women, a second trimester ultrasound offers them the comfort of knowing their little bundle of joy is healthy and everything is developing perfectly.

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