Enhanced first trimester screening or NT/ NB scan
This scan is done between 11 weeks to 13 weeks and 6 days when the fetal length (CRL) is between 45mm to 84mm. This scan is done for:
- Ascertaining the growth of the developing fetus. This is the best time to accurately date the pregnancy.
- To diagnose multiple pregnancy. Approximately 2% of natural conceptions and 10% of assisted conceptions result in multiple pregnancy. Ultrasound scanning can determine if both babies are developing normally and if the babies share the same placenta which can lead to problems in the pregnancy. In such cases it would be advisable to monitor the pregnancy more closely.
- Early detection of major abnormalities. This scan is sometimes also referred to as “EARLY MORPHOLOGY” scan. All the structures have been formed in the fetus by this time and keep on developing as the pregnancy advances. The major abnormalities like acrania, encephalocele, holoprosencephaly, abdominal wall defect, megacystis, open spina bifida, missing limbs, body stalk anomalies, etc can most of the times be identified on third month ultrasound.
- Assessment of risk for chromosomal abnormalities. DNA, the genetic material is contained in the chromosomes within the cells. We all are made up of 23 pairs of chromosomes with one set being derived from each parent. During the process of fertilisation (formation of the baby) sometimes due to some error, somethings might do wrong leading to an error in the number of chromosomes, which can lead to certain syndromes, the most common being Down Syndrome (Trisomy 21).
The risk for chromosomal abnormalities in particular Down syndrome (Trisomy 21), Edward syndrome (Trisomy 18) and Patau syndrome (Trismoy 13) is assessed based on the findings of the scan which include the thickness of the skin and the fluid underneath behind the neck (NT), presence or absence of nasal bone, fetal heart rate, blood flow in Ductus Venosus and blood flow in the fetal heart. It is then combined with the maternal serum biochemistry (Double Marker test) to provide improved accuracy for risk assessment.
Other tests for chromosomal risk assessment:
NIPT (Non invasive maternal cell free fetal DNA testing) This test assesses fetal DNA in maternal blood with predicted detection
- Risk calculation for preeclampsia (high blood pressure) and fetal growth restriction (decreased growth of baby)- based on maternal demographic characteristics, medical and obstetric history, mean arterial pressure (MAP), uterine artery Dopppler and serum biochemistry
- Risk calculation for fetal Neural tube defects (NTDs)
- Risk calculation for preterm delivery (spontaneous delivery before 34 weeks) – based on maternal characteristics , obstetric history and cervical length
The First Trimester Screening (FTS) test has important implications for you and your family and we have explained here how it is performed and what the results mean. Before undergoing this test it is very important that you have discussed this testing procedure.
Who should be undergo this test?
Leading medical professional organizations recommend that all pregnant females should undergo first trimester Down syndrome screening, regardless of age.
When is the test done?
It involves two tests, a blood test and an ultrasound.
Blood Test – It can be done from 10 weeks to 13 weeks and 6 days of pregnancy. It analyzes four chemicals called free beta-human chorionic gonadotropin (hCG) , pregnancy associated plasma protein-A (PAPP-A), placental growth factor (PLGF) and Alpha Feto Protien (AFP) which are found in the blood of all pregnant women.
Ultrasound – It can be performed from 11 weeks through to 13 weeks and 6 days of pregnancy. It is a highly specialized ultrasound. It takes about 20-30 minutes and depends on the fetal position.
How does first trimester screening assess your baby’s risk?
First trimester screening is not a diagnostic/ confirmatory test, which means it cannot tell you with certainity whether your baby has Down syndrome, trisomy 13, or trisomy 18. Instead, the screening provides a probability number for your baby to have Down syndrome, trisomy 13, or trisomy 18.
This probability or chance is calculated on the following criteria:
- Your age
- Nuchal Translucency (NT – amount of fluid behind the neck of the baby on ultrasound)
- Fetal heart rate
- Newer markers – Nasal bone, Blood flow across Tricuspid valve, Blood flow in Ductus Venosus
- Presence or absence of any physical abnormalities on ultrasound
- Level of three proteins (Fß-hCG, PAPP-A and PLGF) in your blood
The screening results can either alert you and your doctor that your baby is at an increased risk for one of these chromosome disorders or be reassuring that your baby is at a lower risk for these conditions.
Women at high risk are offered further tests. It is important to remember that most women will have normal babies, even if they are grouped as high risk.
What are the risks and side effects to the mother or baby?
There are no known side effects or risks associated with first trimester screening to the mother or the fetus. Ultrasound is absolutely safe for the baby and the mother.
What is the accuracy of First Trimester Screening?
This is a screening test and not a diagnostic test. Thus it only gives a probability of having a baby with Down syndrome. A positive test (i.e showing an increased risk) does not mean that your baby has a problem, but only that further diagnostic tests can be considered. Also, a negative or normal result (one that shows a decreased risk) does not mean that the baby will not have a chromosomal abnormality but only means that probability is low..
The first trimester screen’s detection rate for Down syndrome is approximately 95%. There is a 5% false positive rate for the test. Parents should be aware of the possibility of receiving abnormal results and then finding, after further testing, that the baby is normal.
UNDERSTANDING YOUR RESULTS
What does a “screen negative” result mean?
Screen negative means that the probability that your baby has Down syndrome is better than 1:250
It is important to remember that a screening test does not guarantee a normal baby.
What does a “screen positive” result mean?
A screen positive result means that there is an increased risk for Down syndrome, trisomy 13, or trisomy 18 in the pregnancy. A screen positive result does not mean that the fetus is affected with the condition, only that a pregnancy has an increased risk for the condition. Most women with “screen positive” results have healthy babies.
What are the diagnostic tests available?
Diagnostic tests are tests which definitively tell about the baby’s condition. Diagnostic tests will usually involve amniocentesis or chorionic villus sampling, in which a small amount of fluid or tissue from around the baby is taken for chromosomal analysis.
Both tests have a small risk of miscarriage.
The results from the chromosomal analysis may take 2-3 weeks. Usually an interim result may be available in a few days.
Some individuals or couples may elect not to pursue testing or additional testing for various reasons:
- They are comfortable with the results no matter what the outcome
- Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
- Some parents choose not to allow any testing that poses any risk of harming the developing baby
It is important to discuss the risks and benefits of testing thoroughly. We will help you evaluate if the benefits from the results could outweigh any risks from the procedure.