Prenatal screening tests are medical tests conducted to evaluate the overall health of the foetus and to assess the progress of pregnancy. Detection and identification of genetic abnormalities and birth defects are also done during the prenatal tests.
In This Article –
Importance Of Prenatal Screening Tests
All pregnant women need to get their prenatal screening tests every trimester until the birth to gauge the health of the baby and the mother. Some woman who are under high-risk pregnancy, are advised to get a few non-routine tests done to prevent any complications that might not have been picked under routine tests. Post screening, the Ob-gyn runs a few diagnostic tests to confirm the presence of any abnormality or defect that may have been present in the baby. Screening tests only assess the potential risk that may be present in the baby, whereas, diagnostic tests confirm the abnormalities. Thereby further course of action can be advised by the Ob-gyn.
High Risk Factors for Prenatal Screening
- Age>35 years
- Teenage Pregnancy
- History of Genetic Abnormalities
- History of Birth Defects
- History of Recurrent Miscarriage
- Mother suffers from Hypertension, Diabetes, Seizures, STDs, Malignancies
- Previous history of multiple pregnancies, previous baby with genetic/ birth defects
Prenatal Tests Done In 1st Trimester
- Biochemical tests:
Measurement of β-hCG and PAPP-A (Pregnancy Associated Plasma Protein- A) is done in 1st trimester. Normally, β-hCG values are high and PAPP -A values are low. Time of test is during11th– 14th week of pregnancy.
- Ultrasound measurement of nuchal translucency (NT):
NT measures the size of the clear space in the tissue at the back of the baby’s neck in between skin and soft tissue. NT≥3mm is abnormal. It is done to check genetic abnormalities in the baby like Down’s syndrome, cardiac abnormalities and other chromosomal defects. Following the result of the tests, if the baby is in high risk, then diagnostic tests are done. Time of test is between 11th and 14th week of gestation
- Blood tests:
To detect the presence of infections that may be transmitted from the mother to the foetus like Rubella, Syphilis, HIV, Hepatitis-B and to check the Haemoglobin levels for preventing anaemia. Measurement of Rh protein status in the maternal blood is also done as its presence in the baby and absence in the mother may lead to Haemolytic disease of new-born, which is fatal.
- Non-invasive prenatal testing from maternal blood/plasma:
Foetal DNA (mainly cff-DNA) is detected in maternal plasma/ whole blood and is assessed for genetic abnormalities like trisomy 13, 18, 21 etc. A woman tested positive should be referred to genetic counselling and offered invasive prenatal diagnostic tests for confirmation. Time of tests is around 10th week of gestation
- Early Ultrasound scan:
Ultrasound scan is done during early gestational period to confirm pregnancy, rule out abnormal pregnancies like molar pregnancy/ ectopic pregnancy. Dating scan is done early-on to assess the estimated date of delivery (EDD). Thus, the Ob-gyn can schedule subsequent check-ups accordingly. Foetal heart rate (FHR) is also monitored using the ultrasound scan which is usually around 120- 160 bpm. Time of test is Once at the beginning of pregnancy, another between 18th – 20th (in 2nd trimester) week of gestation
- Transvaginal scan:
This scan assesses the vagina, cervix, uterus and ovaries along with the placenta, thus checks the abnormalities in any of these structures. Foetal heart rate can also be monitored. Time of test is between 6th and 10th week of gestation
- Abdominal scan:
This scan assesses the abdominal organs and monitors growth and development of baby. Time of test is between 6th and 7th week of gestation
- Chorionic villus sampling (CVS):
It is an invasive diagnostic test done to diagnose genetic disorders. A sample of chorionic villi tissue is collected under ultrasound guidance to confirm genetic defects. Time of test is between 10th and 13th week of gestation
- Urine analysis A few urine tests are run during prenatal visits for detecting gestational diabetes in pregnant women, infections and preeclampsia. Tests for STDs (HIV, syphilis, gonorrhoea, chlamydia) are done during early prenatal visits to diagnose and prevent their transmission to the baby
- Pap smear to detect cervical cancer may be done during early prenatal visit.
- Regular Blood pressure recordings are taken to diagnose hypertension and preeclampsia.
Prenatal Tests Done In 2nd Trimester
- Triple test and Quadruple tests (Multiple marker tests):
They detect biochemical analytes like MSAFP (maternal serum α-feto protein), hCG and uE3 in the former test and dimeric inhibin-A. They are mainly done to detect trisomy 21 (Down syndrome), neural tube defects and Spina bifida in the baby. Time of test is during 15th and 22nd week of gestation
- Amniocentesis:
It is an invasive procedure done under ultrasonography guidance to obtain amniotic fluid which is subjected for cytogenetic studies. Done to detect genetic defects like trisomy 21 (Down syndrome) Time of test is around 15th – 20th week
- Ultrasound scan:
As mentioned earlier, multiple ultrasound scans are done throughout the period of gestation for assessing the growth and development of baby.
- Glucose challenge test:
It is a screening test done to measure the risk of a pregnant woman to have gestational diabetes. A solution containing 75g of glucose with water is given to the pregnant woman who was previously fasting for 8hours. Following intake, the Glucose levels are measured every hour for 3 hours. Time of test is around 24th – 28th week of gestation
- Foetal doppler ultrasound:
It is an instrument that measures the flow of blood via the blood vessels of the foetus and gives an audio and visual representation of the same. Time of test is around 22nd– 24th and 30th to 34th week of gestation
- Fetoscopy:
An instrument called fetoscope is introduced into the uterus for imaging the baby for any defects and collection of any sample for cytogenetic studies.Time of test is during 18th week of gestation
- Cordocentesis:
An invasive procedure that detects genetic defects in baby along with detection of foetal anaemia, Rh disease, infections, bleeding disorders and foetal blood-gas/ acid-base status. Time of test is beyond 18th week of gestation
Prenatal Tests Done In 3rd Trimester
- Growth scan:
It is a foetal ultrasound scan that is done in the last trimester to assess the growth and development of the baby. The position and amniotic fluid levels are analysed. This test can be done multiple times, the first between 28th and 32nd week of gestation
- Electronic Foetal heart monitoring:
Foetal heart rate is monitored during pregnancy and labour to assess the wellbeing of the baby.
- Group-B streptococcus screening:
GBS screening is done by taking vaginal swabs and cultured later on. GBS infection cause mental retardation, hearing loss, and lungs infections and is very fatal for the baby. Time of test is around 35th – 37th week of gestation
screening and diagnostic tests are done for the early detection and treatment of certain abnormalities and prevent complications during labour. Strictly following advice from the Ob-gyn and attending all the scheduled appointments is important to prevent any unforeseen complications for the mother and baby during labour.
References
https://www.nhs.uk/pregnancy/your-pregnancy-care/screening-tests/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC59263/
https://parenting.firstcry.com/articles/your-ultimate-guide-to-tests-during-pregnancy/
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