If you’ve been visiting your obstetrician (OB) for prenatal care during your first trimester, you’ve already undergone important tests your doctor provides to help ensure your baby is healthy. Depending on what screenings you received during your first trimester, the results from these screenings and your personal health factors, your OB may offer optional blood testing during your second trimester to screen for chromosomal abnormalities and neural tube defects in your baby.

Dr. Imelda Odibo wears a green shirt and smiles at the camera.
Dr. Imelda Odibo

Dr. Imelda Odibo, a maternal fetal medicine specialist who cares for women with high-risk pregnancies at Novant Health Maternal Fetal Medicine in Wilmington, encourages all expectant moms to take advantage of the valuable information these tests can provide.

“We feel like it's a part of taking good care of our patients,” Odibo said.

While some women choose not to receive these optional tests, Odibo emphasizes that these test results allow parents to be prepared for a safe pregnancy and birth. If a chromosomal abnormality causes cardiac defects for the baby, for example, the baby may require a special ultrasound of the heart and surgery shortly after birth. Odibo emphasized that knowing this ahead of time helps the medical team to plan, as well as for the parents to prepare for any special care needs for their child.

“Most women would prefer to know ahead of time if there is an issue, so that they are more prepared to take care of their baby,” she said.

Every pregnancy is different, so your OB or clinician may recommend tests that are different from ones your friends or family members have received. In addition, insurance coverage varies for these tests, so you can inquire with your insurance company or clinician’s office about your plan coverage.

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What are the different second-trimester blood tests and how are they conducted?

Second-trimester blood tests, also known as multiple marker screens, are used to detect abnormalities in your baby by measuring substances in your blood. They may be performed between 15 and 21 weeks of gestation and are conducted with a sample of blood drawn using a needle from your vein – not the baby. This sample is sent to a lab and the results are returned to your clinician’s office, typically within about seven days.

The blood tests your OB may offer include a triple screen (three), quad screen (four), or penta screen (five), each named for the number of substances it measures in your blood. Your OB may also recommend a maternal serum AFP (msAFP) test, which screens for neural tube defects in your baby.

An alternate screening recommendation that your OB may make is called a cell-free fetal DNA test. This examines fetal DNA in your bloodstream. Your doctor may recommend this test if you have had a previous pregnancy with a genetic disorder, you are over the age of 35 or you have another health condition that makes your pregnancy high-risk. The cell-free fetal DNA test is different from the multiple marker screenings because it uses DNA, and can be performed earlier than the second trimester, at 10 weeks and up. The cell-free fetal DNA test is the most accurate screening test for detecting fetal chromosome abnormalities. As with the multiple marker screenings, it is followed with a diagnostic procedure if it reveals abnormal results.

What do these tests detect?

The purpose of these second-trimester blood tests is to screen for chromosome disorders and neural tube defects. A chromosome disorder changes the way a baby’s brain and body develop and may cause physical and cognitive impairments. Chromosome disorders screened for include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).

Triple, quad and penta screens also detect neural tube defects, which occur when the baby’s neural tube, which forms the brain and spine, does not close properly. They do this by measuring the level of alpha-fetoprotein (AFP) level in your blood. For women who receive the cell-free fetal DNA test, AFP may be measured using a separate blood test, or an ultrasound anatomy scan may be used to screen for neural tube defects.

What happens if I receive an abnormal result on my blood test?

If you receive an abnormal result from a multiple marker screening or a cell-free fetal DNA test, your OB will recommend follow-up diagnostic testing to confirm the result. This is because no second trimester blood test is 100% accurate. A detailed anatomy ultrasound is typically recommended to examine how the baby is developing.

“We are looking for any structural abnormalities that might impact how we take care of the baby,” Odibo said.

If abnormalities are suspected early based on the cell-free fetal DNA test, a chorionic villus sampling (CVS) can be done between 10 and 13 weeks to confirm the diagnosis. This test is performed either through the vagina or the mother’s abdomen to collect some of the cells from the tissues surrounding the baby.

Another test used to confirm a chromosomal abnormality in the second trimester is an amniocentesis, which is conducted by using a needle to remove a small sample of amniotic fluid that surrounds the baby in the uterus.

Your OB may refer you to a maternal fetal medicine specialist, like Odibo, who recommends that her patients also work with the genetic counselor on her team. A genetic counselor specializes in examining the details of the test results, reviewing the results with you and supporting you as you make decisions about how your pregnancy will be managed based on the test result. The genetic counselor also reviews your health and family history to help determine the likelihood of other chromosomal abnormalities in future pregnancies.