What a double marker test is....
The double marker test, also known as maternal serum screening, is part of a more comprehensive screening called the first trimester screening. It’s not a definitive test. Instead, it’s classified as a predictive test, which means its results report the likelihood of chromosomal abnormalities.
Specifically, this test screens for blood levels of both free beta-human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein A (PAPP-A).
In a typical pregnancy, there will be either 22 pairs of XX chromosomes in female fetuses or 22 pairs of XY chromosomes in male fetuses.
A trisomy is a chromosomal condition in which there are extra chromosomes, such as the following:
- Down syndrome. This common trisomy is also referred to as trisomy 21 because there’s an extra copy of chromosome 21.
- Trisomy 18 and trisomy 13. These common chromosomal abnormalities involve an extra copy of chromosome 18 (Edward’s syndrome) or chromosome 13 (Patau’s syndrome).
Levels of hCG and PAPP-A may be either higher or lower than “normal” in pregnancies with these chromosomal abnormalities.
However, blood levels alone don’t produce your results. Instead, the blood test is used along with an ultrasound called a nuchal translucency (NT) scan, which examines the clear tissue at the back of your baby’s neck.
Why a double marker test is done
The first trimester screening — double marker test and NT scan — isn’t mandatory. That said, the screening (and others like the cell-free DNA test) is recommended if you’re over the age of 35 or may have an elevated risk of chromosomal issues, such as if you have a family history of certain conditions.
It’s important to remember that the result only tells you whether there’s an increased risk of trisomies. It doesn’t definitively determine whether your baby has any abnormalities.
Before deciding whether you want a double marker test, you might ask yourself what the results would mean to you in the long run.
· Would knowing about possible abnormalities ease or worsen your anxiety?
· Would you want to go for more invasive testing if you receive a result that indicates heightened risk?
· Would the results change how you’d manage your pregnancy?
It’s all about personal choice and your own health history, so there’s really no right or wrong answers to your questions.
Timing of the double marker test
There’s a relatively slim time window during which the double marker test can be performed. Your healthcare provider will have you make an appointment for sometime near the end of your first trimester or possibly very early in the second trimester.
More specifically, you’ll have your blood drawn sometime between weeks 11 and 14.
Standard results for the double marker test
You’ll receive a low-, moderate-, or high-risk result.
Low-risk (“screen-negative”) is considered a “normal” result and means that there’s a low probability of your baby having chromosomal abnormalities.
If your result is in the normal range, you’ll only be recommended for further testing if there’s some other indicator (like family history, age, etc.) or if you desire to learn more for some other reason.
Still, a low-risk result doesn’t always mean your baby won’t have an issue. And it’s important to note that the first trimester screening only looks at markers for Down syndrome, trisomy 13, and trisomy 18 — not other conditions.
Abnormal results for the double marker test
If your screening comes back as moderate- or high-risk (“screen-positive”) for abnormalities, you may choose to go over your result with a genetic counselor to learn more.
Your result can be confirmed with more definitive and sometimes more invasive tests, such as noninvasive prenatal testing (NIPT), amniocentesis, or chorionic villous sampling. While some of these tests carry some risk to your pregnancy, they provide a definitive result.
Since the double marker test is done early on in your pregnancy, the information gives you time to make important decisions about further testing, medical treatments, and the overall management of your pregnancy and delivery.
Knowing your risk may also help you prepare for the potential you’ll have a child with special needs and allow you to find the support you’ll need.
Double marker test vs. NT scan
For the most accurate results, the double marker test (blood test) and NT scan (ultrasound) are used together in the first trimester screening. The information gathered from both tests is what gives the result of a low-, moderate-, or high-risk of abnormalities.
When performed without the double marker test, the NT scan is less effective at detecting potential abnormalities.
The NT scan allows your healthcare provider to use sound waves to collect a real-time image of your baby. It’s performed around the same time as your double marker test.
At that point in pregnancy — late in the first trimester — your healthcare provider can measure the size of the clear area on the back of your baby’s neck. Your doctor will also assess the development of the nasal bone, which may be another indicator of a trisomy.
These measurements are combined with your blood results and age-related risk. They’re all calculated together to produce your baby’s potential risk of Down syndrome, trisomy 13, or trisomy 18.
What You'll Find Out from an NT Scan During Pregnancy
Pregnancy screenings take place during the first, second, and third trimesters. The first trimester screening is a type of prenatal testing that provides your doctor with early information about your baby’s health — namely your baby’s risk for chromosome abnormalities.
A nuchal translucency (NT) scan screens your baby for these abnormalities. This test is typically scheduled between weeks 11 and 13 of pregnancy.
What’s the purpose of an NT scan?
An NT scan is a common screening test that occurs during the first trimester of pregnancy. This test measures the size of the clear tissue, called the nuchal translucency, at the back of your baby’s neck.
It’s not unusual for a fetus to have fluid or clear space at the back of their neck. But too much clear space can indicate Down syndrome, or might show another chromosome abnormality like Patau syndrome or Edwards syndrome.
Our body cells have many parts, including a nucleus. The nucleus holds our genetic material. In most cases, the nucleus has 23 pairs of chromosomes, which are equally inherited from both parents.
Individuals born with Down syndrome have an extra copy of chromosome 21. Down syndrome, which can’t be cured, causes developmental delays and distinct physical characteristics.
These include:
- a small stature
- eyes with an upward slant
- low muscle tone
This condition affects 1 in every 700 babies born in the United States. It’s one of the most common genetic conditions.
Patau syndrome and Edwards syndrome are rare and often fatal chromosome abnormalities. Unfortunately, most babies born with these abnormalities die within the first year of life.
When is an NT scan scheduled during pregnancy?
The clear space in the back of a developing baby’s neck can disappear by week 15, so an NT scan should be completed in the first trimester.
This test can also include blood work to measure your levels of plasma protein and human chorionic gonadotropin (HCG), a maternal hormone. Abnormal levels of either may indicate a chromosome problem.
How does an NT scan work?
During the screening, your doctor or a technician will take an abdominal ultrasound. You could alternatively have a transvaginal test, where an ultrasound probe is inserted through your vagina.
An ultrasound uses high frequency sound waves to create an image from inside your body. From this image, your doctor or technician measures the translucency, or clear space, at the back of your baby’s neck. They can then enter your age or date of birth in a computer program to calculate the risk of your baby having an abnormality.
An NT scan cannot diagnose Down syndrome or any other chromosome abnormality. The test only predicts the risk. Talk to your doctor about available blood tests. They also can help assess your baby’s risk.
As with any prediction, the accuracy rate varies. If you combine an NT scan with blood testing, the screening is about 85 percent accurate for predicting the risk of Down syndrome. If you don’t combine blood testing with the scan, the accuracy rate drops to 75 percent.
No special preparation is required for an NT scan. In most cases, testing is completed in about 30 minutes. During the scan, you’ll lie down on an exam table as the technician moves an ultrasound wand over your stomach.
The ultrasound pictures may be easier to read if you have a full bladder, so your doctor may recommend drinking water about one hour before your appointment. The ultrasound tech needs access to your lower abdomen, so make sure you wear comfortable clothing that makes it accessible.
Results from the scan may be available on the same day of testing, and your doctor may discuss the findings with you before you leave. It’s important to remember that receiving an abnormal result from an NT scan doesn’t necessarily mean that your baby has a chromosome problem. Similarly, normal test results can’t guarantee that your baby won’t be born with Down syndrome.
This test isn’t perfect. There’s a 5 percent false-positive rate. In other words, 5 percent of women tested receive positive results, but the baby is fine. After a positive result, your doctor may suggest another blood test called prenatal cell-free DNA screening. This test examines fetal DNA in your bloodstream to assess your baby’s risk for Down syndrome and other chromosome abnormalities.
Screening vs. diagnostic testing
It can be frightening to receive inconclusive or positive results from an NT scan. Keep in mind that an NT scan can only predict your baby’s risk: It doesn’t offer a definitive answer about chromosomal abnormalities. An NT scan is a screening test, not a diagnostic test.
There are differences between screening and diagnostic testing. The purpose of a screening test is to identify risk factors for a particular disease or condition. Diagnostic testing, on the other hand, confirms the presence of a disease or condition.
How to diagnose an abnormality
To diagnosis a chromosome abnormality, ask your doctor about diagnostic testing. Options include an amniocentesis, which is when a needle is inserted through your stomach into the amniotic sac to retrieve a fluid sample. Amniotic fluid contains cells that provide genetic information about your baby.
Another option is chorionic villus sampling. A sample of your placental tissue is removed and tested for chromosome abnormalities and genetic problems. There’s a small risk of miscarriage with both tests
The takeaway
An NT scan is a safe, noninvasive test that doesn’t cause any harm to you or your baby. Keep in mind that this first trimester screening is recommended, but it’s optional. Some women skip this particular test because they don’t want to know their risk. Talk to your doctor if you experience anxiety, or are worried about how the results might affect you.
Crown Rump Length (CRL) on Ultrasounds
Crown-rump length (CRL) is an ultrasound measurement that is used during pregnancy. The baby measured, in centimeters, from the top of their head (crown) to the bottom of their buttocks (rump).
The limbs and yolk sac are not included in the measurement. The CRL can be measured starting around six or seven weeks of pregnancy up until 14 weeks
What Is Crown-Rump Length?
CRL may be useful in calculating gestational age. With this gestational age, doctors can estimate your potential due date. The earlier the first ultrasound is performed, the more accurate the baby's gestational age will be.
Once the fetus has developed past 14 weeks, head circumference, biparietal diameter, and femur length measurements are used to determine how the baby is progressing.
The length of the umbilical cord is typically the same as the CRL throughout pregnancy.
Uses
Once the fetus's CRL surpasses 7 mm, a heartbeat should be detected by ultrasound. If no heartbeat or cardiac activity is detected, then the pregnancy is likely a missed miscarriage.
A missed, or silent, miscarriage typically occurs without the normal miscarriage symptoms.
The placenta may continue to supply hormones, which can mask the outward signs of a miscarriage.
Women with a mean sac diameter (MSD) of less than 5 mm greater than the CRL are more likely to experience a first-trimester miscarriage even if the baby has a normal heart rate.
Decreased crown-rump length can also diagnose chromosomal anomalies like trisomy 18 (Edwards syndrome) and other trisomies associated with growth restriction.
The embryo and fetus float in the amniotic fluid inside the uterus of the mother usually in a curved posture resembling the letter C. The measurement can actually vary slightly if the fetus is temporarily stretching (straightening) its body. The measurement needs to be in the natural state with an unstretched body which is actually C shaped. The measurement of CRL is useful in determining the gestational age (menstrual age starting from the first day of the last menstrual period) and thus the expected date of delivery (EDD). Different babies do grow at different rates and thus the gestational age is an approximation.
Recent evidence has indicated that CRL growth (and thus the approximation of gestational age) may be influenced by maternal factors such as age, smoking, and folic acid intake. Early in pregnancy gestational age 8 weeks, it is accurate within about +/- 5 days but later in pregnancy due to different growth rates, the accuracy is less. In that situation, other parameters can be used in addition to CRL. The length of the umbilical cord is approximately equal to the CRL throughout pregnancy.
Gestational age is not the same as fertilization age. It takes about 14 days from the first day of the last menstrual period for conception to take place and thus for the conceptus to form. The age from this point in time (conception) is called the fertilization age and is thus 2 weeks shorter than the gestational age.
Thus a 6-week gestational age would be a 4-week fertilization age. Some authorities however casually interchange these terms[citation needed] and the reader is advised to be cautious. An average gestational period (duration of pregnancy from the first day of the last menstrual period up to delivery) is 280 days. On average, this is 9 months and 6 days.
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