The first-trimester screening is a prenatal test that gives early information about a baby’s risk of certain chromosomal conditions such as extra sequences of chromosome 18 (trisomy 18) or Down syndrome (trisomy 21).
This screening has two steps:
a blood test to measure levels of two pregnancy-specific substances in the blood of the mother (human chorionic gonadotropin or HCG and pregnancy-associated plasma protein-A)
an ultrasound exam to measure the size of the clear space at the back of the baby’s neck or nuchal translucency. In most cases, this screening is done between the 11th to 14th weeks of pregnancy
Your doctor can gauge your risk of carrying a baby with trisomy 18 or Down syndrome by using your age and the results of the ultrasound and blood test.
Your doctor will suggest another test that is more definitive if the results showed that your risk level is moderate of high.
2 Reasons for Procedure
The reason for a first-trimester screening test is to check if your baby has a trisomy 18 or Down syndrome.
Trisomy 18 can cause severe delays in your baby and is often fatal by the age of 1 while Down syndrome can cause lifelong impairments in the social and mental development and various physical concerns, but this does not evaluate the risk of neural tube defects such as spina bifida.
First-trimester screening can be done earlier than other prenatal screening tests so you can have more time to make decisions regarding your course of pregnancy, further diagnostic tests, management during and after delivery and medical treatment.
The other screening test can be done later in pregnancy such as quad screen which is a blood test that can evaluate your risk of carrying a baby with Down syndrome or trisomy 18 and neural tube defects, and mostly this is done within the 15th to 20th weeks of pregnancy.
Some of the doctors like to combine quad screen with first-trimester screening which is called integrated screening which can improve the detection rate of Down syndrome.
First-trimester screening does not say whether your baby has one of the said conditions, it will only indicate if you are at risk of carrying a baby with Down syndrome or trisomy 18.
This test may influence on how you will manage your pregnancy or if you will get anxiety depending on the results. You may also want to consider a more invasive test.
The first-trimester screening is a routine prenatal screening test that poses no potential risk of pregnancy complications or miscarriage.
4 Preparing for your Procedure
There are no special things to do in preparing for the first-trimester screening test.
You can do normal things such as eating and drinking before both the ultrasound exam and blood test.
5 What to Expect
Here you can find out what to expect from a first-trimester screening test.
First-trimester screening includes an ultrasound exam and a blood draw.
In an ultrasound exam, you will lie on your back on a table and your doctor or an ultrasound technician will place a plastic device which sends and receive sound waves in your abdomen.
These sound waves will be converted into images on a monitor and your doctor will use these images to measure the size of the clear space in the tissue at the back of your baby’s neck.
In a blood test, your doctor will get a sample of your blood by inserting a needle into a vein in your arm and he will send this to the laboratory for analysis.
6 Procedure Results
Your doctor will use the results of the first-trimester screening test and the blood test and at the same time your age to gauge your risk of carrying a baby with trisomy 18 or Down syndrome.
The first-trimester screening result will be positive or negative and also as a probability such as a 1 in 250 risks of carrying a baby with Down syndrome.
This will identify about 85 percent of women who are carrying a baby with Down syndrome and about 5 percent of women who have a false-positive result which means that the test result is positive but the baby doesn’t have Down syndrome.
This test will only indicate your overall risk of carrying a baby with Down syndrome or trisomy 18 but a high-risk result does not mean that your baby will be born with one of these conditions or a low-risk will not guarantee that your baby will not have one of these conditions.
If you have a positive result, your doctor will suggest additional testing such as:
Prenatal cell-free DNA (cfDNA) screening – this test will determine if your baby is at risk of Down syndrome, trisomy 18 or trisomy 13 by examining the fetal DNA in the maternal bloodstream. This screening can also provide information about fetal gender and screen for other chromosome problems.
Chorionic villus sampling (CVS) – this is done during the first trimester that can diagnose Down syndrome by getting a sample of tissue from a placenta, and this test poses a small risk of miscarriage.
Amniocentesis – this is don during the second trimester that can diagnose Down syndrome or neural tube defects by getting a sample of amniotic fluid in the uterus for testing, this also poses a small risk of miscarriage.
Your doctor will help you understand the test results and what it means for your pregnancy.
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