Most people are aware of some, but often not all of the tests that are being offered or performed on them during pregnancy. Understanding your options in advance will lead to more meaningful discussions with your doctor or health care team. Genetic testing in pregnancy is optional, and learning about what information each of these tests gives you allows you the opportunity to vocalize what information you do or do not want to know.
Below is a brief breakdown of all the tests that are offered which give you information on the genetic health of your baby.
First Trimester Screen with NT Scan
This is a blood test often done in conjunction with an ultrasound at around 12 wks gestation that measures the back of the baby's neck. Results from the blood test and the ultrasound measurement are put together to give you a risk estimate for some of the more common chromosome conditions.
Key Points:
Does not look at all chromosomes
Gives you a risk estimate, not a yes/no answer
Lower detection rate and higher false positive rate than NIPT (see below)
A higher than normal NT measurement can be indicative of heart defects or other genetic syndromes.
A positive (abnormal or increased risk) result does not mean your baby has the condition.
Non Invasive Prenatal Screening
Also referred to as NIPS or NIPT. This is a blood test done after 9 wks gestation that can tell you with a higher detection rate that first trimester screening, the chance for baby to have some of the more common chromosome conditions.
Key Points:
Doesn't look at all chromosomes
Doesn't give you a yes/no answer about the chromosome conditions, but can tell you if you're low or high risk
Not invasive, ie no risk of complications from the test, but there is risk that you may miss something (when compared to diagnostic tests- more on this below)
Some labs offer NIPT for microdeletions and microduplications. The chance that a positive result is a true positive (known as Positive Predictive Value) for these conditions can be low. It’s best to check these stats out with the lab that’s doing the test to have a better understanding of how to interpret the results.
A positive (abnormal or increased risk) result does not mean your baby definitely has the condition. All results should be confirmed via a diagnostic test such as CVS or Amnio.
Carrier Screening
This is a blood test done usually first on mom that can tell you if you carry a change on a gene for certain inherited genetic conditions. The conditions screened for all impact childhood health and are not adult-onset conditions. For most of the conditions screened for, if both mom and dad have a change in the gene for the same condition, then there is a 25% chance that baby could be affected with that condition (Autosomal recessive inheritance). There are some conditions where if mom is a carrier, there is a 50% chance for a child to inherit the change (X-linked inheritance). The severity of the condition may vary between males and females for X-linked inheritance conditions.
Key Points:
Often there are large panels with 200+ conditions screened for
Many of these conditions are super rare, but can have significant health implications. Having knowledge of increased risk is helpful to know for potential early treatment and planning.
A negative (normal) result does not eliminate all risk, but significantly reduces the risk
It is not unusual to be told you are a carrier, especially if you’ve been screened on a larger panel. In fact, most people who are found to be a carrier state that they have no idea this condition ran in their family.
A carrier is typically healthy and doesn’t show any symptoms (this is why you generally have no idea that you’re a carrier until you actually get tested).
This test can be done even before you are pregnant since it is genetic info specific to you (and not the pregnancy).
2nd Trimester Screening, AFP Measurement
This is a blood test that is done typically around 16-18 wks gestation. The alpha-fetoprotein (AFP) is a protein made by the baby that crosses the placenta into mom's blood. Elevated levels of AFP can indicate certain structural abnormalities in the baby, such as neural tube defects.
Diagnostic Tests
These are tests referred to as chorionic villus sampling (CVS) or amniocentesis (amnio). They will give you information about the number and structure of all the chromosomes. You can also get super detailed information from additional testing referred to as microarray analysis where the chromosomes are analyzed for microdeletions and microduplications. Both tests are invasive in that they are taking a sample of either the chorionic villi from the placenta (in CVS) or the amniotic fluid surrounding baby (in amnio). While both are safe procedures, they do come with a risk of complications that could lead to miscarriage.
Key Points:
Gives you the highest detection rate compared to screening tests
Much more comprehensive than any of the screening tests (like first trimester screening or NIPT).
Has risk for complications that could lead to miscarriage. Each center that performs these tests will have their own numbers for risks. On average, I’ve seen numbers anywhere from 1/200-1/700.
Level II Ultrasound
Detailed ultrasound performed around 18-20 wks gestation. At this point, baby is large enough to take a detailed head to toe look at all the structures. Measurements are taken of all the bones, the fluid around baby is checked and the flow of blood through the heart is assessed. If any structural changes are noted, you may be referred to a genetics professional to further evaluate if the findings could be associated with a genetic syndrome or condition.
The above is just a quick breakdown of the 6 tests that are typically offered to pregnant woman beginning in early pregnancy. There's no right or wrong way to approach testing; but it is important to figure out what options would be the best for the type of information that matters to you. For some people, they know exactly how they would want to proceed, but for most they're a bit ambivalent. Chatting with a genetic counselor can be very informative in learning more about what test may be the optimal choice for you. At our telehealth practice at FiND Genetics, we take the time to understand your questions, personalize information, and guide you so that you can get to answers that you feel confident about.