First trimester screening is a routine prenatal test that provides initial information about the fetus’s risk of certain chromosomal conditions also called fetal aneuploidies. Normally every human being cell contains a total of 46 chromosomes. But in an aneuploidy condition, there can be an abnormal number of chromosomes, it can be 45 or 47 usually instead of 46.
The chromosomal condition specifically Down syndrome (one extra copy of chromosome 21 then called, 21 trisomies), Edward Sydrom, an additional copy of chromosome 18 sequences (then called chromosome 18 trisomies) and Patau Syndrom, an extra copy of chromosome 13 ( called chromosome 13 trisomies) is checked.
First trimester screening, also known as first-trimester combined testing, has two stages:
- This test measures the level of two pregnancy-specific substances in the mother’s blood – pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG)
- An ultrasound examination to measure the size of the empty space in the tissue at the back of the baby’s neck (called the nuclear translucency)
“Typically, screening for the first trimester is done between 11 and 14 weeks of pregnancy.”
Using your age and blood test and ultrasound results, the doctor can assess the risk of carrying a child with Down syndrome or trisomy 18.
If the results show that the risk level is moderate or high, then the patient can choose screening of the first trimester with a second test that is more definitive.
During the First Trimester Screening test, you can also take a preeclampsia screening test. Preeclampsia is a condition when you have high blood pressure and protein content in the urine. It needs to be checked early unless it can posses a threat to pregnant mothers as well as babies after birth.
What is the purpose of the first-trimester pregnancy screening test?
The first trimester is examined to assess the risk of carrying a child with Down syndrome. The test also provides information about the risk of trisomy 18 and 13.
Down syndrome causes lifelong impairments in mental and social development, as well as many physical problems. Trisomy 18 causes more severe delays and is usually fatal at 1 year of age. Trisomy 13 may disrupt normal development, which further can cause multiple and complex organ defects.
First-trimester screening does not assess the risk of neural tube defects, such as spina bifida.
Since the first-trimester screening can be done earlier than other prenatal screening tests, you will have early pregnancy results. This will give you more time to make decisions during other clinical trials, course of pregnancy, medical treatment and management, and after delivery. If your child has an increased risk of Down syndrome, you will have more time to prepare for the possibility of caring for a child with special needs.
Other screening tests may be done later in pregnancy. An example is a quadruple screen, a blood test typically performed between 15 and 20 weeks pregnant. Quadruple screening can assess your risk of carrying a child with Down syndrome or chromosome 18 trisomy, as well as neural tube defects such as spina bifida. Some healthcare professionals chose to combine the results of the first-trimester examination with a quadruple exam. This is called an integrated screening. This may improve the detection rate of Down’s syndrome.
First-trimester screening is optional. The test results only indicate if you are at risk of carrying a child with Down syndrome or chromosome 18 trisomy, and not whether your child actually has one of these conditions.
Before the screening, don’t think about what the results will be for you. Consider whether screening can cause any concerns or it will manage your pregnancy differently depending on the outcome. You can also consider what level of risk would be sufficient for you to select a more aggressive follow-up test.
What are the risks of taking this test?
First-trimester screening is a routine antenatal screening test. Screening does not pose a risk of miscarriage or other pregnancy complications.
How to prepare for this test?
You don’t need to do anything special to get ready for your first-trimester Screening/ antenatal test. You can eat and drink normally before a blood test and ultrasound test.
What pregnant lady can expect during this test?
The first-trimester examination includes normal blood collection from the arm by a phlebotomist and an ultrasound examination.
During a blood test, a member of the paramedical team inserts a needle into the vein of your arm and takes a blood sample. Then this collected blood sample is sent to a laboratory for analysis. You can return to your daily normal activities just after this test.
For the ultrasound examination, you will lie on your back on the examination table. Your doctor or ultrasound technician will place a transducer – a small plastic device that sends and receives sound waves on your abdomen. The reflected sound waves will be digitally converted into images on the monitor. Your doctor or technician will use these pictures to measure the size of the empty space in your child’s throat.
Ultrasound does not hurt and you can immediately return to your normal activities.
What can be the possible result?
Your doctor will use the results of your age and blood tests and ultrasound to assess the risk of carrying a child with Down syndrome or trisomy 18. Other factors – such as a previous pregnancy with Down syndrome – may also affect your risk.
Screening results in the first trimester are given as positive or negative and because of the probability of 1 to 250 risk of carrying a child with Down syndrome.
The sensitivity ( means which correctly identifies the down syndrome cases) of the first-trimester screening test is 85 percent. About 5% of women have an incorrect or false-positive result, meaning the test result is positive, but the child does not have Down syndrome.
When considering your test results, remember that first-trimester screening only indicates the overall risk of carrying a fetus with Down syndrome or trisomy 18. A low-risk result does not guarantee that the fetus will not have one of these conditions. Similarly, a high-risk outcome does not guarantee that the fetus will be born with one of these conditions.
If you have a positive test result, your doctor and a genetics professional will discuss your options, including additional tests.
(Disclaimer: Information provided in this piece of article is purely for educational purposes only. All results must be clinically correlated with the patient’s data to make an accurate diagnosis.)
- V Zournatzi, J., 2020. A Prospective Two Years Study Of First Trimester Screening For Down Syndrome. [online] PubMed Central (PMC).
- Healthline. Prenatal Screening Tests. [online]