Genetic Testing During Pregnancy: How Does It Work?

Key Takeaways

Genetic testing during pregnancy can test for chromosomal abnormalities, a cleft lip or cleft palate, spina bifida, cystic fibrosis or spinal muscular atrophy, omphalocele and gastroschisis, sickle cell anemia, and more.
Some reasons a parent might get a genetic test for their unborn child are a family history of genetic disorders, previous miscarriage or stillbirth, a previous baby was born with a genetic abnormality, the mother is above 35 years old, or there was an abnormality on the ultrasound scan.
Prenatal genetic screening in pregnant women is not foolproof. There can be false negative and false positive results.

Many genetic abnormalities can be diagnosed prenatally (before a baby is born). If you have a family history of a genetic disease or other risk factor, your obstetrician may have recommended getting genetic testing during pregnancy. Please continue reading to find out how it works.

Why would a parent typically get a genetic test for their unborn child?

A parent might get a genetic test for their unborn child if:

They or their partner have a family history of genetic disorders.
There is a history of pregnancy loss (miscarriage) or stillbirth.
A previous baby was born with a genetic abnormality.
Advanced maternal age (above 35 years).
The obstetrician suspects an abnormality on routine prenatal screening (fetal ultrasound scan).
A genetic screening test is positive (this may prompt a diagnostic test).

Healthcare providers can arrange a meeting with a genetic counselor to help parents understand the screening tests and diagnostic tests that can be done during pregnancy.

Genetic counseling is always recommended before any prenatal genetic testing during pregnancy so that parents can understand the tests and the implications of the results.

What are the different types of genetic tests during pregnancy?

There are two main categories of prenatal tests done to detect genetic disorders: screening tests and diagnostic tests.

Prenatal screening tests

They tell the risk of certain inherited disorders in the fetus.

Carrier screening

This is offered to expectant parents. It can also be done before a pregnancy. Tests are done on a blood or tissue sample (for example, a cheek swab). Carrier screening detects whether the parents have a gene for certain genetic disorders. It is usually done for single-gene disorders such as sickle cell disease, cystic fibrosis, Tay-Sachs disease, and fragile X syndrome.

Prenatal screening tests

Prenatal genetic screening tests include ultrasound exams of the fetus and tests on maternal blood samples. A prenatal screening test can detect aneuploidy (an extra chromosome or a missing chromosome) and certain birth defects in the heart, brain, abdomen, and face.

First-trimester screening tests

These are done between weeks 10 to 13 of the pregnancy. They include a blood test to measure two substances and an ultrasound. The ultrasound includes a nuchal translucency test to measure the thickness of the fetus’s neck at the back. An abnormal result is linked to an increased risk of trisomy 21 (Down syndrome), a form of aneuploidy, and birth defects of the heart, skeleton, and abdominal wall.

Second-trimester screening

This includes a quadruple or quad test. It is a blood test that measures four substances and is done at 15-22 weeks for trisomy 18 (Edward syndrome), trisomy 21 (Down syndrome), and neural tube defects. Second-trimester screening also includes an ultrasound at 18-22 weeks for major birth defects of the facial features, limbs, spine, heart, brain, and abdomen.

Combined screening

This is a combination of tests done during first and second-trimester screening. It often gives more accurate results, but the results are only available when the second trimester has been completed.

Cell-free fetal DNA test

Cell-free DNA screening measures the fetal DNA that leaks from the placenta into the maternal bloodstream. It is a blood test that can detect a higher risk of trisomy 13 (Patau syndrome), trisomy 18 (Edward syndrome), trisomy 21 (Down syndrome), and abnormalities in the number of sex chromosomes. Cell-free DNA screening may be offered at 10 weeks of pregnancy. A positive screening test is usually followed up with diagnostic testing such as amniocentesis.

Prenatal diagnostic tests

Diagnostic genetic testing tells, with as much certainty as possible, whether the fetus has a genetic disorder. This is in contrast to a screening test, which tells the risk of certain birth defects and genetic disorders. There are two types of prenatal diagnostic tests for genetic conditions:

Amniocentesis

A genetic amniocentesis test is usually done at 16-20 weeks of pregnancy, but it can be done right up to the time of delivery. It involves using a thin needle to withdraw a small amount of amniotic fluid that surrounds the fetus. This is done using ultrasound guidance. The analysis and results can take anywhere from 2 days to several weeks, depending on the type of information desired.

Chorionic villus sampling (CVS)

This type of prenatal diagnostic testing involves taking a small tissue sample from the placenta. It can be done earlier than amniocentesis, at around 10-13 weeks of pregnancy.

Frequently Asked Questions

What genetic testing is done during pregnancy?

Non-invasive prenatal testing includes ultrasound and blood tests that provide information about the baby’s health. They include screening and diagnostic tests. Screening tests tell the risk of certain genetic conditions. However, a screening test won’t tell if the fetus has a particular condition. A diagnostic test tells, with as much certainty as possible, if a certain genetic condition is present. Healthcare providers may recommend diagnostic testing if screening test results raise suspicions of fetal abnormalities.

What does genetic testing test for in pregnancy?

Genetic testing during pregnancy can test for chromosomal abnormalities such as Down syndrome, fragile X syndrome, or Turner syndrome; a birth defect of the facial features such as a cleft lip or cleft palate; neural tube defects such as spina bifida; a genetic condition such as cystic fibrosis or spinal muscular atrophy; physical defects of the abdominal wall such as omphalocele and gastroschisis; and many others.

What genetic testing is done at 12 weeks?

Genetic testing done at 12 weeks includes screening tests (first-trimester screening tests and cell-free DNA screening of fetal cells). It also includes diagnostic tests (chorionic villus sampling).

How risky is prenatal genetic testing?

Amniocentesis can cause slight bleeding and leakage of amniotic fluid, which usually stop on their own. There is also a very small risk of miscarriage with amniocentesis. The chance of pregnancy loss is slightly higher with chorionic villus sampling (CVS) compared to amniocentesis.

Can genetic testing during pregnancy be wrong?

Prenatal genetic screening in pregnant women is not foolproof. There can be false negative and false positive results. This means the tests may fail to detect an abnormality that is present (false negative), or the tests may detect an abnormality that is, in fact, not present (false positive).