Ultrasounds during Your Second Trimester

By Joanne Stone, Keith Eddleman, Mary Duenwald

An ultrasound (also referred to as a sonogram) exam is an incredibly useful tool that allows you and your doctor to see the baby inside your uterus. A device called a transducer emits sound waves. The sound waves are reflected off the fetus and converted into an image that appears on a monitor.

You can see almost all the structures in the fetus’s body, and you can see the fetus moving around and performing all its normal activities — kicking, waving, and so on. The best time to view the baby’s anatomy is around 18 to 22 weeks.

An ultrasound exam doesn’t hurt. Your practitioner spreads gel or lotion over your abdomen, and then moves the transducer around through the gel. A full bladder isn’t necessary because the amniotic fluid surrounding the fetus provides the liquid needed to transmit the sound waves to create a clear or detailed picture. Picture quality varies, depending on maternal fat, scar tissue, and the fetus’s position.

A doctor (an obstetrician, a perinatologist, or a radiologist) or an ultrasound technologist may perform the ultrasound. Sometimes a technologist does a preliminary exam, and the doctor comes in later to check on images or review the printed pictures.

[Credit: Kathryn Born, MA]
Credit: Kathryn Born, MA

What an ultrasound can reveal

Ultrasound is like a checkup for the fetus. It can provide information about the following:

  • Number of babies

  • Gestational age

  • Rate of fetal growth

  • Fetal position, movement, and breathing exercises (the fetus moves its chest and abdomen as if it were breathing air)

  • Fetal heart rate

  • Amount of amniotic fluid

  • Location of placenta

  • General fetal anatomy, including the identification of some birth defects

  • The baby’s sex (after 15 to 16 weeks), although depending on the position of the fetus, the sex may be difficult to visualize

Typically, the examiner measures the fetus first and then studies its anatomy. The extent and degree of detail of the exam varies from woman to woman and doctor to doctor. A detailed ultrasound can examine these structures:

  • Arms and legs

  • Bladder

  • Brain and skull

  • Face

  • Genitalia

  • Heart, chest cavity, and diaphragm

  • Kidneys

  • Spine

  • Stomach, abdominal cavity, and abdominal wall

Reasons for having an ultrasound

Whether and how often you need an ultrasound depends on your particular risk factors, your doctor’s preferences, and your insurance coverage. Some doctors recommend that all women have an ultrasound exam at about 20 weeks; others feel that it’s unnecessary if your risks for having problems are low. Multiple ultrasound examinations may be needed if any of the following conditions arise:

  • You’re carrying twins or more.

  • Your doctor suspects that the baby is too small or too large for its age.

  • Your doctor suspects that you have too little or too much amniotic fluid.

  • You’re at risk for preterm labor or incompetent cervix.

  • You have diabetes, hypertension, or other underlying medical conditions.

  • You’re bleeding.

  • Your doctor wants to do a biophysical profile, which is an assessment of fetal well-being that looks at movement, breathing exercises, amniotic fluid volume, and fetal tone (ability to flex the muscles).

Recently, doctors have also used ultrasound to get an accurate measurement of the cervix (the uterus’s opening) in women at risk for preterm delivery or incompetent cervix. Your practitioner places a transducer in the vagina to measure the cervix’s length and check the appearance of the lower part of the uterus.