Birth defects affect 1 in every 33 babies born in the US, according to the CDC. While that may not seem like a lot in the grand scheme, consider that every 4.5 minutes a baby is born in the U.S. with a birth defect. That translates to about 120,000 babies each year [*]. Worldwide, the percentage of babies born with birth defects is closer to 6% [*].
What causes birth defects is largely a mystery, though known risk factors include smoking or drinking alcohol, taking certain drugs, the presence of maternal infection such as Zika virus, and high fever. But for the bulk of birth defects, scientists point to a complex and unknown interaction between the parents’ genes, the mother’s age, health issues like diabetes, behavior, and environmental factors.
The good news is, there are a wealth of tests that can help parents make informed decisions on the road to parenthood — even before the baby is conceived.
Here, we look at the types of tests and examine their pros and cons.
What Is Prenatal Testing?
Prenatal tests provide you and your doctor a view into your baby’s development while also screening for certain genetic issues. While no testing can guarantee a healthy baby, prenatal testing can help identify pregnancy complications and birth defects at the earliest possible moment. In some cases, these prenatal tests can be lifesaving to mother and baby.
What Are the Types of Prenatal Tests?
Prenatal testing divides into two categories: screening and diagnostic. The main difference is that screening tests are designed to quantify your risk of having a child with a birth defect while diagnostic testing can offer more definitive answers and test for more abnormalities. Screening tests rely on bloodwork and ultrasound imaging while diagnostic tests require tissue or cells.
Prenatal Screening Tests
Screening tests are performed via blood tests, urinalysis, and ultrasound. During pregnancy, these tests monitor the baby’s progress and detect abnormalities characteristic of potential disorders or infections.
The amount of genetic information doctors can get from a simple blood draw is expanding rapidly, thanks to advancements in testing [*].
For instance, prior to conception, parental carrier screening can help couples determine what genetic anomalies they carry and calculate the likelihood of them passing them on to their offspring. This information could help them determine whether to go with IVF to mitigate such risks through pre-implantation genetic diagnosis.
For parents who conceive via the traditional route, screening results coupled with other risk factors — the mother’s age, the parents’ ethnic background, the family history of genetic disorders — are used to calculate the odds that the fetus might be born with certain genetic disorders, such as Down syndrome, cystic fibrosis, Tay-Sachs disease, or sickle cell anemia. There are no risks to the fetus from these genetic screening tests.
- A blood test performed in the first trimester checks for certain markers — proteins or hormones — indicative of Down syndrome. Bloodwork is also performed early on to test for Rh factor and diseases like rubella, hepatitis B and C, HIV, and tuberculosis.
- A nuchal translucency ultrasound is offered in the first trimester. This test measures increased fluid or thickness at the back of the baby’s neck indicative of Down syndrome. Together, the blood and ultrasound tests are remarkably effective at detecting Down syndrome and aneuploidy (chromosome abnormality) [*]. Abnormal nuchal translucency can also be indicative of risk of other birth defects such as Turner syndrome or congenital heart disease.
- A blood test in the second trimester screens for three or four markers indicative of aneuploidy. Tests done later in pregnancy may also include a glucose screening and GBS (Group B streptococcus) screening.
- A combined test (sometimes called an integrated test) uses the blood test and ultrasound from the first trimester together with the second trimester blood test to calculate a risk rating.
Screening Test Risk & Accuracy
These standard tests now reach a detection rate of up to 88–96% for Down syndrome and up to 85–95% for trisomy 18, depending on whether screening is performed in the first or second trimester of pregnancy, or both [*].
There is the potential for a false positive of around 5%. If the mother is pregnant with multiples, a blood test will not be as reliable because the markers from a Down syndrome fetus may be harder to detect.
There are no risks to the fetus, or the mother associated with screening tests.
Non-Invasive Prenatal Testing for Genetic Screening
Non-invasive prenatal testing (NIPT), also known as cell-free fetal DNA screening (CFDNA or CFFDNA), is the most recent addition to the genetic screening toolkit. Available since 2011, NIPT is the most accurate screen for chromosome abnormalities, and in certain countries like the Netherlands, it has become the standard of care for all pregnant women.
Unlike the other screening tests that look for hormone levels or proteins, this test relies on bits of DNA fragments from the placenta circulating in the mother’s blood (assuming that the placenta DNA matches that of the baby). It can be done any time after 10 weeks’ gestation and is usually done before 21 weeks.
It is used to detect Down syndrome, Trisomy 13 and Trisomy 18, sex chromosome abnormalities, and the fetus’s Rh factor. It has a very high detection rate for common aneuploidies (98-99%) and low false positives (.1-.2%). It does not detect other abnormalities and insurance coverage for it may be conditional, but if your screening reveals an issue you want to formally diagnose, it makes sense to do it since it’s noninvasive and highly accurate.
Out-of-pocket costs for NIPT range from $800-$2,000 in the U.S [*]. Contact your insurer to determine your coverage or review The American College of Obstetrics and Gynecology NIPT insurance coverage overview here.
Prenatal Diagnostic Tests
Prenatal diagnostic tests tell parents if their baby is affected with a genetic disorder, as opposed to screening tests which calculate their risk of having a baby affected.
For patients at increased risk due to maternal age, abnormal screening results, genetic markers, or family history, diagnostic testing is indicated. These tests require tissue or cells to be collected and are invasive.
While some women choose to do this after a genetic screening indicates a high risk, others may bypass the screening step if they want a definite answer earlier in their pregnancy. Diagnostic tests also can detect other types of chromosomal disorders that genetic screening tests cannot.
There are three diagnostic tests:
- Chorionic villus sampling (CVS): CVS is performed in the first trimester, between 10-13 weeks of pregnancy. It tests placenta tissue, which is taken either transcervically or transabdominally, i.e., via the cervix akin to a Pap smear or by using a needle inserted through the mother’s abdomen into the uterus. CVS cannot detect neural tube defects like spina bifida, so if that is a concern, an ultrasound or genetic amniocentesis might be a better choice.
- Amniocentesis (Amnio): Amnio is a second trimester test performed at 15 weeks or later in which a needle is inserted through the mother’s abdomen to extract fluid surrounding the baby. The amniotic fluid contains fetal cells used for testing.
- Percutaneous umbilical blood sampling (PUBS): PUBS, also known as cordocentesis, is the most accurate diagnostic method but is less often used. It involves taking a sample of fetal blood from the umbilical cord via the uterus. A needle is guided by ultrasound and then introduced into a blood vessel (usually the vein) of the umbilical cord. The blood can be tested for genetic disorders, blood conditions (PUBS is most often used to test for anemia) and infections. It can also be used to confirm the results of CVS or amnio but it cannot be performed until 18-22 weeks’ gestation [*]. PUBS may also be used to treat a critically ill fetus by delivering blood or medication through the umbilical cord [*].
Diagnostic Test Risk & Accuracy
Both CVS and Amnio carry minimal risk of complications like infection, premature rupture of membranes, or even miscarriage. CVS has a .5-1% risk of miscarriage, according to the Mayo Clinic. Amnio risk of miscarriage is 1 in 400, or .25% [*]. PUBS carries higher risk of complications or miscarriage, 1-2%, so it is usually only done when a diagnosis cannot be made via amnio or CVS. According to the Mayo Clinic, the risk numbers for PUBS may be skewed by the fact that many fetuses are already quite ill when the test is done at 18-22 weeks. [*]
For women of any age, at least half of first trimester miscarriages occur due to a chromosome abnormality in the baby [*].
How Common Are Birth Defects From Genetics?
To date, more than 7,000 different birth defects of genetic or partially genetic origin have been identified.
The prevalence of chromosomal abnormalities in clinically recognized miscarriage is greater than 50%. Fetuses with aneuploidy account for 6-11% of all stillbirths and neonatal deaths [*].
According to the March of Dimes, experience from high-income countries shows that nearly 70% of birth defects can either be prevented or the affected children can be offered care that could save their lives or reduce the severity of disability. These interventions include appropriate treatment, particularly surgery, and prevention, especially before conception or in very early pregnancy.
Do Doctors Recommend Prenatal Genetic Testing?
Doctors invariably look at patient outcomes when they make recommendations, and the facts here speak for themselves: The United States reported a remarkable 46% decline in infant mortality rates from birth defects over the period 1980 to 2001, and much of this reduction can be attributed to improvements in diagnosis, care, and prevention. Other high-income countries have reported similar declines. And that was 20 years ago, before NIPT and other advancements in prenatal testing [*].
The objective of prenatal genetic testing is to detect health problems that could affect the woman, fetus, or newborn and provide the patient and her obstetric care provider with the information they need to make fully informed decisions about their pregnancy.
The American College of Obstetricians and Gynecologists urges doctors to make the aims and limitations of prenatal testing clear to patients and ensure the tests are aligned with the patient’s risks, reproductive goals, and preferences [*].
The Pros and Cons of Genetic Testing
To know or not to know… this is the question. Here are the pros and cons:
Pros of Genetic Testing
- Reassure patients when results are normal.
- Empower parents to make fully informed decisions to manage their pregnancy, even before conception.
- Empower doctors to identify and treat (where feasible) fetal conditions in utero, significantly improving the chance of survival and reducing the need for major surgery after birth [*].
- Optimize neonatal outcomes by ensuring an appropriate delivery location and personnel to care for an affected infant or manage a potentially complicated birth.
- Ensure parents are prepared, emotionally and medically, to care for an infant with special needs.
- Allow for the opportunity to terminate a pregnancy.
Cons of Genetic Testing
- Minor risk of complications or even miscarriage associated with invasive diagnostic testing.
- The emotional toll of pregnancy management when your fetus is diagnosed with a birth defect.
- The possibility of a false positive in a screening test (usually resolved with diagnostic testing).
- Cost of certain tests where insurance coverage is unavailable or limited.
- Lack of availability to underserved communities.
What to Do if Your Child Is Predisposed
If a parent is known to carry a heritable genetic disease or carrier genetic testing reveals this, pre-implantation genetic diagnosis and IVF can ensure that the disease is not passed on to the offspring, but IVF is expensive, and not all insurance providers cover it.
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Screening or testing for birth defects is a personal choice, not a requirement. There are times when, for emotional or religious reasons, parents prefer not to know if they are at risk of having a child with a birth defect or whether their child will have one.
Conversely, there are women who would very much welcome the chance to better manage their pregnancy risks but cannot afford it or simply don’t have access.
Meanwhile, parents who opt for in vitro fertilization often opt for some genetic screening and pre-implantation genetic diagnosis to avoid costly multiple cycles and failed pregnancies.
Whatever the emotions surrounding it, prenatal genetic testing is a miracle that improves lives. Though not all birth defects can be detected or avoided, prenatal genetic screening and diagnostic tests can help parents determine whether to continue the pregnancy, prepare for a child with special needs, or perhaps avoid the defect or mitigate its severity while the fetus is still in the womb. And that’s remarkable.