A Guide to Prenatal Tests and Scans | Health Plus

A Guide to Prenatal Tests and Scans | Health Plus

Congratulations! Now that you have a bun in the oven, you will be seeing your doctor frequently – and even more so during the final trimester – to ensure your pregnancy is a healthy one.

There are a series of prenatal tests and scans available to check on the well-being of both mother and baby during pregnancy but not all tests are necessary. Your doctor will be able to advise you on the ones you need.

Apart from the routine tests that are carried out at every antenatal visit (eg. blood pressure, weight, and urine dipstick for sugar and protein), the following tests are also performed if necessary.

Trimester Months Pregnant Weeks Pregnant Test (*not compulsory) Procedure / Purpose
1 0 0 – 4    
1 5 – 8 Viability ultrasound To confirm that pregnancy is developing normally
Blood tests
(Any time during 1st trimester)
To check for full blood count, blood group, hepatitis, rubella, and sexually transmitted diseases
2 9 – 12 Dating ultrasound To calculate expected due date
*Non-invasive Prenatal Testing (NIPT)
(from Week 9)
Blood test that detects foetal DNA in mother's blood sample
*Chorionic Villus Sampling (CMV)
(Week 11 – 12)
Invasive diagnostic test that involves extraction and biopsy of small sample of placental tissue
*One-Stop Clinic for Assessment of Risk for foetal anomalies (OSCAR)
(Week 11 – 14)
To calculate risk for common chromosomal disorders and other genetic disorders in foetus
To diagnose chromosomal and other genetic disorders in foetus
Ultrasound scan to assess the thickness of skin in the neck of the foetus, and blood test from mother
Calculates risk for abnormalities or genetic diseases in foetus
2 3 14 – 17 *Amniocentesis
(Week 16 – 20)
Invasive diagnostic test that involves extraction and testing of small sample of amniotic fluid
To diagnose chromosomal and other genetic disorders in foetus.
4 18 – 21 Detailed foetal anomaly scan
(Week 19 – 22)
To check for structural anomalies in foetus, and other details like placental position
5 22 – 25 Oral Glucose Tolerance Test
(Week 24 – 28)
3 blood tests (Fasting, then drinking a 75g glucose drink, and 2 other blood tests 1 and 2 hours later)
Screens for gestational diabetes in the mother
3 6 28 – 30 Regular and doppler ultrasound
(Weeks 28 – 42)

Cardiotocograph
(Any time during 3rd trimester and labour)
Monitor growth of foetus and flow in blood vessels supplying blood to foetus
Continuous electronic foetal monitor to record foetal heartbeat, and track uterine contractions
To assess well-being of foetus
8 35 – 38 Group B Streptococcus test A vaginal swab to test for bacteria known as Group B Streptococcus

First Trimester Tests

Blood tests

Blood test
The essential blood tests include:

  • Full blood count – to check for anaemia and thalasaemia, a common genetic blood disorder
  • Blood group and Rhesus status – to check for rhesus compatibility between mother and foetus
  • Hepatitis B
  • Rubella (German Measles)
  • VDRL (Syphilis)
  • HIV (Human Immunodeficiency Virus)

Other blood tests may also be done depending on the mother’s ethnic background and medical history. These include screening for infections like Toxoplasmosis (a parasitic disease) and Cytomegalovirus; and screening for carrier status of genetic disorders like cystic fibrosis, sickle cell disease and Tay-Sachs disease. Results for blood tests may take up to a week.

Viability and dating ultrasound scans

An early viability ultrasound scan from about 6 – 8 weeks of gestation can confirm that the pregnancy is developing normally and will identify the foetal heartbeat.

From about 8 – 10 weeks, dating the pregnancy can be performed by ultrasound and is accurate to within a week. This can ensure the most accurate calculation of the expected due date (EDD).

Screening tests for Down Syndrome (Trisomy 21) and other chromosomal problems

Down Syndrome test
These tests are not compulsory but all pregnant women should be offered them regardless of age. Several options for this type of screening are available. Note that screening tests are not diagnostic – this means that while they are mostly accurate, an ‘abnormal’ result will need to be confirmed by more invasive methods of testing (see section below on CVS and amniocentesis).

1. One-stop clinic for assessment of risk for foetal anomalies (OSCAR)

This consists of 2 tests. First, a certified operator will perform an ultrasound scan to assess the thickness of skin in the neck of the foetus (Nuchal Translucency). Next a blood sample is taken from the mother. The combined results are analysed with a proprietary computer algorithm software and a resultant ‘risk score’ is calculated. The test is performed between 11 and 14 weeks, and is about 90% accurate. Test results are typically available within the same day.

2. Non-Invasive Prenatal Testing (NIPT)

This refers to a group of tests that detect foetal DNA (also known as cell free DNA) in a sample of blood taken from the mother. In actual fact, this DNA arises from the placenta and not actually from the foetus itself. There is therefore a small chance that the results may not be 100% accurate.

These tests can be done from 9 weeks onwards and they all screen for the common chromosomal disorders: Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome). In addition, they can also determine gender, and other genetic disorders like triploidy and microdeletions. While they cannot detect every genetic disorder, they are highly accurate for the most common ones like Down Syndrome (97 – 99%).

There are several companies that offer the test, and each one offers slightly different screening panels for the various genetic disorders. The companies include Harmony, MaterniT21 Plus, Panorama, EasyDNA and iGene.

Diagnostic tests for chromosomal or genetic disorder

Amniocentesis
These are invasive tests and are usually indicated for the following:

  • women who have had a previous baby with a chromosomal/genetic disorder and wish to have a test with 100% accuracy
  • women who have a high-risk result from OSCAR or NIPT and wish to have a confirmatory test

1. Chorionic Villus Sampling (CVS)

CVS can be performed earlier, at 11 – 12 weeks, and involves inserting a biopsy needle through the mother’s abdomen under ultrasound guidance into the placenta to withdraw a small amount of placental tissue for testing. The whole procedure takes a few minutes and is relatively painless. Results typically take 2 – 3 weeks.

2. Amniocentesis

Amniocentesis is usually performed later, between 16 – 20 weeks and also involves inserting a hollow bore needle through the abdomen under ultrasound guidance. The needle is directed into a pool of amniotic fluid surrounding the foetus and a small amount of amniotic fluid is removed for testing. Results are available after 2 – 3 weeks.

Both CVS and Amniocentesis are invasive tests and carry a small risk of causing miscarriage (CVS 0.5 – 1%; Amniocentesis 0.25 – 0.5%).

Second Trimester Tests

Detailed foetal anomaly scan

Detailed foetal anomaly scan
This is usually performed between 19 – 22 weeks. This test looks for structural anomalies in the foetus that can occur even though there are no genetic abnormalities detected. In particular, the scan checks for any abnormal heart structures, facial clefts, head, abdomen and limb size, placental position, doppler blood flow, cervical length, and the amount of amniotic fluid present.

Oral glucose tolerance test

This test, which screens for gestational diabetes, is usually performed between 24 – 28 weeks. Gestational diabetes is a type of diabetes that affects some women during pregnancy. While most cases will resolve after delivery, women with gestational diabetes are at higher risk of developing Type 2 diabetes later in life.

It involves 3 blood tests. You will begin the test in a fasting state, and drink a 75g glucose drink. This is followed by 2 more blood tests 1 and 2 hours later. Results are usually available within 1 – 3 days.

Third Trimester Tests

Regular and Doppler Ultrasound Scans

Ultrasound scan
From late in the second trimester and going into the third trimester, some women may be required to undergo repeat ultrasound scans to monitor the growth of the foetus. This usually also includes Doppler ultrasound scans that check the flow in the blood vessels supplying blood to the foetus.

Group B Streptococcus (GBS) test

This is a group of bacteria that is normally present in the genital area of around 30% of women. GBS generally does not cause any problems, but can affect the baby as it passes through the birth canal during delivery. A vaginal swab is taken, usually around 35 weeks, and if it is positive for GBS, antibiotics are usually prescribed.

Cardiotocograph (CTG)

This is a routine test performed during labour to continuously monitor baby’s heartbeat as well as the frequency and intensity of any uterine contractions. In the third trimester, a CTG may also be ordered by the doctor to monitor the well-being of the baby.

 

Article reviewed by Dr Quek Swee Chong, obstetrician and gynaecologist at Gleneagles Hospital

References

Glucose screening tests during pregnancy. (n.d.). Retrieved December 1, 2018 from https://medlineplus.gov/ency/article/007562.htm

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