A prenatal test that measures levels of four substances in a pregnant woman's blood is called the quad screen or quadruple marker test, the quad test or the second-trimester screen.
The four substances are:
human chorionic gonadotropin (HCG) which is a hormone made by the placenta
alpha-fetoprotein (AFP) which is a protein made by the developing baby
inhibin A which is another hormone made by the placenta
estriol which is a hormone made by the placenta and the baby's liver
This is mostly done between the 15th to 18th weeks of pregnancy but can be done up to the 20th week.
This is done to check if you are going to have a possibility of having a baby with chromosomal conditions.
The alpha-fetoprotein part of the test can help evaluate the chance for neural tube defects, such as spina bifida, and abdominal wall defects, such as omphalocele.
The quad screen can offer reassurance that there is a decreased chance for Down syndrome, trisomy 18, neural tube defects and abdominal wall defects if your risk level is low but if it indicates increased chance you may want to have additional screening.
Here are the most common reasons to undergo a quad screen.
This is done to evaluate following conditions such as:
trisomy 18 – this causes severe developmental delays and anatomic abnormalities. Trisomy 18 is often fatal by age 1
Down syndrome – this causes lifelong intellectual disability and developmental delays and, in some people, health problems
spina bifida – this causes defects in the spinal cord and in the bones of the spine because it occurs when a portion of the neural tube fails to develop or close properly
anencephaly – this results in an underdeveloped brain and an incomplete skull
This is done during the second trimester or if first-trimester screening, which involves a blood test and an ultrasound exam, wasn't available. This is the most commonly used screening in the second trimester.
Prenatal cell-free DNA screening is a new screening method that your doctor might recommend in place of quad screening.
Test results only indicate whether you have an increased chance of carrying a baby with certain chromosomal abnormalities, such as Down syndrome, or neural tube defects, such as spina bifida, not a diagnostic test.
Your doctor will recommend additional testing to make a diagnosis if your screening test is positive, but a negative quad screen doesn't guarantee that the baby won't have a chromosomal abnormality.
3 Potential Risks
The quad screen is a routine prenatal screening test. It has no risk of miscarriage or other pregnancy complications but this may cause anxiety especially if the result is positive.
4 Preparing for your Procedure
In order to prepare for the quad screen, your doctor may ask you to meet with a genetic counselor to discuss the test. You can normally eat and drink before the test.
Here you can find out what to expect from your quad screen.
By inserting a needle into a vein in your arm, your doctor may get a sample of blood which is done by your doctor.
He will send the blood to the lab and you may return to your normal activities after the procedure.
6 Procedure Results
Understanding the results of your quad screen will be made possible by your doctor. The quad screen measures levels of AFP, HCG, estriol and inhibin A in a pregnant woman's blood.
To evaluate your chance of carrying a baby who has certain chromosomal conditions, neural tube defects or abdominal wall defects, your doctor will use your age at the estimated time of delivery and the results of the quad screen.
Quad screen results are given as a probability, such as a 1 in 500 risks of carrying a baby who has Down syndrome.
Factors that can affect the substances measured by a quad screen include:
Your doctor may recommend an ultrasound to verify the baby's gestational age and confirm the number of babies if the results are positive.
The quad screen correctly identifies about 80 percent of women who are carrying a baby who has Down syndrome and about 5 percent of women have a false-positive result.
A negative screen result doesn't guarantee that your baby won't have one of these conditions as well as a positive screen result does not say that your baby will be born with one of the said conditions.
For secondary testing:
Prenatal cell-free DNA screening – this evaluates whether your baby is at risk of Down syndrome, extra sequences of chromosome 13 (trisomy 13), extra sequences of chromosome 18 (trisomy 18) or a sex chromosome abnormality, such as Turner syndrome.
Targeted ultrasound – your doctor may recommend this but not effective for Down syndrome.
Chorionic villus sampling – this is used to diagnose chromosomal conditions, such as Down syndrome but this poses a slight risk of miscarriage and isn't useful in detecting neural tube defects, such as spina bifida.
Amniocentesis – this can be used to getting a sample of amniotic fluid in the uterus to diagnose both chromosomal conditions, such as Down syndrome, and neural tube defects, such as spina bifida but poses a slight risk of miscarriage.
Discuss with your doctor about the result of your test.
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