Pregnant women routinely receive screening tests at different intervals to check the health of their developing babies, and an abnormal test result can put a damper on their excitement. Abnormal results mean that the baby is at increased risk of a specific condition, and doctors will recommend diagnostic testing to learn more.

“There are limitations to what any particular screening test can identify,” said Emily Seet, MD, medical director of Maternal-Fetal Medicine at Tarzana. “A diagnostic test confirms if there is an actual genetic abnormality. Every pregnant patient has options to evaluate whether or not their fetus is at risk for a chromosomal or genetic disorder, or birth defect.”

Doctors offer one of two diagnostic tests—chorionic villus sampling (CVS) or amniocentesis. Which test is given depends on how far along the pregnancy is: CVS is performed between 10 and 13 weeks, and amniocentesis is done after 15 weeks.

“There are small differences in what the two tests can find, but in most cases, the choice between them depends on the timing of a patient’s abnormal screening test results or when they would like to have diagnostic testing done,” Seet said.

Both CVS and amniocentesis look at cells directly from the pregnancy. CVS samples cells from the placenta, and amniocentesis samples the amniotic fluid, which carries cells from the fetus.

Patients with abnormal results from an ultrasound or blood test screening are not the only ones to seek diagnostic testing. Those with a family history of a particular genetic condition also may benefit. Pregnant patients who are 35 or older may seek diagnostic testing because their fetus is at increased risk of Down syndrome. The American College of Obstetrics and Gynecology recommends that all pregnant women be offered diagnostic testing, regardless of their age or other risk factors.

Genetic counselors can help you decide whether diagnostic testing is appropriate.

“They can be involved early in the pregnancy based on family history,” Seet said, “and if we identify any ultrasound abnormalities, we will involve genetic counselors. They’re right there with us to inform patients about how the conditions may affect the pregnancy or a child later in life.”

If testing finds that a genetic condition is present, genetic counselors can also provide patients with information about the condition and what it might mean for their baby. This knowledge can help patients make informed decisions about their pregnancy care.

Read: Do Children Need Genetic Testing?

OB-GYNs may refer patients to maternal-fetal medicine specialists for diagnostic testing. CVS and amniocentesis are both performed in doctor’s offices while patients are awake. During each test, the doctor inserts a thin needle to remove cells, which are examined for abnormalities.

“Both are done with ultrasound guidance, so we can see our needle placement,” Seet said. “CVS is commonly transcervical or through the cervix. Amniocentesis is done through the abdomen.”

Both tests increase the risk of temporary bleeding, cramping or infection. Most patients don’t experience serious side effects, but there is about a 1 in 500 risk of miscarriage.

“If someone is anxious about the procedure, it usually helps when I explain to them how the procedure is done and what to expect,” Seet said. “For example, because we use ultrasound guidance, we are able to target exactly where to collect the amniotic fluid specimen.”

Immediately after CVS or amniocentesis, patients are told to take it easy and avoid heavy lifting.

“Within a few days, they should be able to return to normal activity,” Seet said.

A laboratory examines the sampled cells, looking for gene mutations or chromosomal abnormalities. Any variation of these can lead to genetic syndromes—including Down syndrome, cystic fibrosis or Tay-Sachs disease. It typically takes two to three weeks for patients to receive their test results.

“Waiting for the results can be a challenging time,” Seet said. “It is important to remain hopeful, but it can be a time when the patient feels sad, scared or unsure.”

If the test results are normal, the fetus doesn’t have the suspected condition. If diagnostic testing shows that a developing baby has a syndrome or condition, maternal-fetal medicine specialists may perform an ultrasound to further assess the situation. Genetic counselors and maternal-fetal medicine doctors will speak with pregnant patients and their partners about the test results.

“We support the patients and help walk them through the process, providing medical information about the condition,” Seet said. “We connect them with resources to help them understand what to expect and how to prepare for life after birth.”