Prenatal Testing and Scans: A Guide

Photo of author

By BarneyBaker

Prenatal scans and tests can be used to assess the health of mother and baby throughout pregnancy. However, not all tests are required. Your doctor can help you choose the right ones.

If your doctor feels they are necessary, these tests may be added to the regular tests you have at every antenatal appointment.

First Trimester Tests

  • Blood tests

These blood tests are:

  • A complete blood count is required to rule out anaemia or thalasaemia. This is a common genetic blood disorder.
  • To check for compatibility between mother & foetus based on Rhesus status and blood group
  • Hepatitis B
  • Rubella and German Measles
  • Syphilis and other venereal diseases research laboratory (VDRL), tests
  • Human Immunodeficiency Virus (HIV)

Based on the mother’s medical history and ethnic background, other blood tests may be performed. These include screening for parasitic diseases like toxoplasmosis and cytomegalovirus. They are also used to screen for carriers of genetic disorders such as cystic fibrosis and sickle cell disease. The results of blood tests can take up to one week.

Viability and dating ultrasound scans

A pregnancy ultrasound scan at 6-8 weeks gestation can confirm normal development and identify the foetus’ heartbeat.

Ultrasound can be used to date the pregnancy from 8-10 weeks. It is accurate up to one week. This allows for the most precise calculation of the expected due day (EDD).

Screening for Down Syndrome (Trisomy 21), and other chromosomal disorders

Although these tests are not mandatory, they should be available to all pregnant women regardless of their age. There are many options available for screening. Screening tests are not diagnostic. This means that they can be inaccurate and an abnormal result may need to be confirmed with more invasive testing (see the section below on CVS or amniocentesis).

1. One-stop clinic to assess foetus anomalies risk (OSCAR).

There are two types of tests. A certified operator will first perform an ultrasound scan in order to determine the thickness of the skin around the neck of the foetus. This is called a nuchal translucency. Next, a mother is asked to give a blood sample. A proprietary algorithm software analyses the combined results and calculates a ‘risk score. It takes between 11-14 weeks to complete the test, which is approximately 90% accurate. Usually, the test results are available within one day.

2. Non-invasive Prenatal Testing (NIPT).

This is a set of tests that can detect foetal DNA in blood samples taken from the mother. This DNA is actually derived from the placenta, and not from the foetus. Therefore, there is a chance that the results might not be 100% accurate.

The tests are available from 9 weeks and can screen for common chromosomal disorders such as Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome) and Trisomy13 for Patau Syndrome. They can also identify gender and other genetic disorders such as microdeletions and triploidy. They are able to detect most genetic disorders, including Down Syndrome, but they can only be as accurate as 97-99%.

The test is offered by several companies, each offering a different panel for screening genetic disorders. These companies are Harmony, MaterniT21 Plus Panorama, EasyDNA, and iGene.

  • Diagnostic tests for genetic disorders or chromosomal problems
  • These tests are non-invasive and are often indicated for the following:
  • Women who have had a baby with a chromosomal/genetic condition in the past and want to get a test that is 100% accurate
  • Women who have received a high-risk OSCAR or NIT result and want to have a confirmatory testing
  1. Chorionic Villus Sampling – CVS

CVS can also be done earlier at 11-12 weeks. Under ultrasound guidance, a biopsy needle is inserted through the mother’s abdomen to draw a small amount placental tissue. It is painless and takes only a few minutes. The results typically take between 2 and 3 weeks.

  1. Amniocentesis

The amniocentesis procedure is performed usually later, typically between 16 and 20 weeks. It involves the inserting of a hollow bore needle into the abdomen using ultrasound guidance. For testing, the needle is inserted into an amniotic fluid pool surrounding the foetus. After 2 to 3 weeks, results are available.

Both CVS (0.5-1%) and Amniocentesis (0.25% – 0.5%) are invasive tests that can cause miscarriage.

  • Second Trimester Tests
  • Detail foetus anomaly scan

This test is typically performed between 19 and 22 weeks. The scan looks for structural abnormalities in the foetus, even if there are no known genetic abnormalities. The scan focuses on abnormalities in the heart, facial clefts and head size. It also checks for placental position and doppler blood flow.

Oral glucose tolerance test

The test screens for gestational diabetics between 24-28 weeks. Gestational diabetes can affect some women during pregnancy. Most cases resolve by the time of delivery. However, gestational diabetes is more common in women who are pregnant.

There are 3 blood tests. The test will be started in a fasting state. You will then consume a 75g glucose beverage. The test is then followed by two more blood tests, one hour and two hours later. Results usually arrive within 1 to 3 days.

  • Third Trimester Tests
  • Doppler and regular Ultrasound Scans

Some women may need to have repeat ultrasound scans starting in the second trimester. Doppler ultrasound scans are used to check blood flow and supply blood to the foetus.

Test for Group B Streptococcus (GBS)

GBS is a group that’s normally found in the genital region of about 30% of women. Although GBS is not usually a problem, it can cause problems for the baby when it enters the birth canal. A vaginal swab, which is usually done at 35 weeks, is taken. If it’s positive for GBS antibiotics are often prescribed.

Cardiotocograph (CTG)

This routine test is performed during labor to monitor the baby’s heartbeat and detect any uterine contractions. A CTG may be ordered by a doctor in the third trimester to monitor the baby’s well-being.