A to Zs of Prenatal Tests and Scans

With a steadily growing bun in the oven, parents-to-be can expect to meet their doctors a lot more frequently!

While not all tests are necessary, there are a series of prenatal tests and scans available to check on the well-being of both mother and baby throughout the 9-months of pregnancy. Read on to find out more about these tests.

First Trimester

Blood Tests

Blood tests include:

  • Full blood count to check for anaemia and thalassemia, a common genetic blood disorder

  • Blood group and Rhesus status – to check for rhesus compatibility between mother and foetus

  • Hepatitis B

  • Rubella or German Measles

  • Syphilis and other venereal disease research laboratory (VDRL) tests

  • Human Immunodeficiency Virus (HIV)

Depending on the mother’s ethnic background and medical history, other blood tests may also be performed. These include screening for infections like toxoplasmosis, parasitic disease and cytomegalovirus - a common virus related to chickenpox and herpes simplex. Other screenings include for carrier status of genetic disorders like cystic fibrosis, sickle cell disease and Tay-Sachs disease.

Viability and Dating Ultrasound Test

A viability ultrasound scan from 6-8 weeks of gestation can confirm if the pregnancy is developing normally and will identify the foetal heartbeat.

From about 8-10 weeks, the expected due date (EDD) can be most accurately calculated to within a week!

Screening Test for Down Syndrome or Other Chromosome Problems

While not compulsory, all pregnant women are recommended to perform this particular screening.

  1. One-Stop Clinic for Assessment of Risk for foetal anomalies (OSCAR)

OSCAR consists of 2 tests altogether. Firstly, a certified operator will perform an ultrasound scan to assess the thickness of skin in the neck of the foetus. This is particularly known as nuchal translucency.

Next, a blood sample is drawn from the mother. Combined results are analysed with a propriety computer algorithm software and a ‘risk score’ is calculated. The test is performed between 11-14 weeks and is about 90% accurate. Results are usually available within the same day.

  1. Non-Invasive Prenatal Testing (NIPT)

NIPT can be done from week 9 of the pregnancy onwards and they screen for the common chromosomal disorders such as Trisomy 21 for Down Syndrome, Trisomy 18 for Edwards Syndrome and Trisomy 13 for Patau Syndrome.

They can also determine gender and other genetic disorders like triploidy and microdeletions. Despite not being able to detect every genetic disorder, they contain between 97%-99% accuracy and are highly accurate for common ones like Down Syndrome.

Diagnostic Tests for a Chromosomal or Genetic Disorder

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.

  1. 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

Detailed Foetal Anomaly Scan

This scan is usually performed between 19-22 weeks. This scan tests for structural anomalies in the foetus that may still occur even when there are no genetic abnormalities detected. More specifically, it checks for the presence of 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 particular test involves 3 blood tests and screens for gestational diabetes - a type of diabetes that affects pregnant women - usually performed between the period of 24-28 weeks. While most cases are resolved after delivery, women who have gestational diabetes are at a higher risk of developing Type 2 diabetes later on in life.

Third Trimester

Regular and doppler ultrasound scans

Some women may be required to undergo repeated ultrasound scans to monitor the growth of the foetus from late in the second trimester and going into the third trimester. Doppler ultrasound scans that check the flow in the blood vessels supplying blood to the foetus are included.

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 the 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.

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