Second Trimester Prenatal Testing: An Ob-Gyn Shares What To Expect

Prenatal testing is important to monitor you and your baby’s health, but exactly what is involved in these screenings?

Here, an obstetrician-gynecologist talks about what to expect during your second trimester prenatal visits, including what tests may be done, when they usually happen, what they look for, how they’re performed and what the results may mean.

When is the second trimester?

Before we can discuss second trimester prenatal visits, we need to answer the question, “When does the second trimester start?” According to Dr. Sudheer Jayaprabhu, an ob-gyn at Christus Health in Texas, most definitions of second trimester have it stretching from week 13 to week 28 of the pregnancy.

Prenatal testing during the second trimester

Quad screen: This voluntary blood test performed between weeks 16 and 18 measures several hormones produced during pregnancy. These tests, also called multiple markers, calculate the potential risk for Down syndrome, neural tube defects like spina bifida and Trisomy 18, according to Stanford Medicine. These tests aren't always accurate, but are used to see who might need more testing.

If there is an abnormal result,” Jayaprabhu says, “genetic counseling and ultrasound are usually performed by a Maternal Fetal Medicine specialist.” Amniocentesis (placing a needle under ultrasound guidance into the uterus and withdrawing amniotic fluid for evaluation) can be performed. However, this test comes with some risk of miscarriage.

“I always counsel my patients to think through the entire process," Jayaprabhu says. "I have had patients who get an abnormal blood test and then don’t want the amniocentesis due to the risks of miscarriage. It leads to a lot of anxiety from this point till delivery.”

If one of these birth defects is found, there is no pre-birth treatment except a termination of the pregnancy.

Ultrasound: During weeks 18 to 22, an ultrasound screening for abnormalities in fetal body structure is usually performed, the Cleveland Clinic says. Ultrasound will be used to take images inside your uterus, to make sure the fetus is growing normally. You may also learn the sex of the fetus. Some of the specific areas they look at include:

• Heart
• Brain, neck and spine
• Kidneys and bladder
• Arms and legs
• Hands, fingers, feet and toes
• Lips, chin, nose, eyes and face
• Chest and lungs
• Stomach and intestines

The ultrasound technician will also:

• Listen to the fetal heart rate for abnormal rhythms
• Check the umbilical cord for blood flow and where it attaches to the placenta
• Look at the placenta to make sure it’s not covering your cervix (placenta previa)
• Check your uterus, ovaries and cervix
• Measure the amount of amniotic fluid

If any abnormalities are found, Jayaprabhu explains the patient will be seen by the Maternal Fetal Medicine specialist with the goal of providing the best outcome for the baby and the mother.

Glucose tolerance test: During weeks 24 to 28, screening for gestational diabetes (diabetes during pregnancy) is performed.

Jayaprabhu describes the testing.

“Usually the screening is performed by giving the non-fasting patient a 50-gram glucose drink and then checking the blood sugar one hour afterwards. If the patient has a blood sugar of greater than 135, then a three-hour glucose test is performed. This test includes checking a fasting blood sugar and then giving the patient a 100-gram glucose drink. This is followed by checking sugars at one, two and three hours after the drink.”

If only one of your readings is abnormal, your doctor may suggest some changes to your diet and/or test you again later in the pregnancy, the American Pregnancy Association says. If two or more of your readings come back abnormal, you’ll be diagnosed with gestational diabetes and your doctor will talk to you about a treatment plan. Identifying and treating gestational diabetes is important as it can lead to larger babies, difficulty during delivery, C-sections, stillbirths and high blood pressure in the mother.

Knowing what to expect when you go to your second trimester appointments can lower your stress levels and allow you to fully enjoy the second trimester of your pregnancy.

Resources:

Sudheer Jayaprabhu, MD, ob-gyn, Christus Health, Texas

Stanford Medicine: Second Trimester Prenatal Screening Tests

Cleveland Clinic: 20-Week Ultrasound (Anatomy Scan)

American Pregnancy Association: Glucose Tolerance Test