Antenatal Screening Guide for Mothers by Dr Ryan Lee

Antenatal screening tests can help detect pregnancy complications and allow a mother to receive the right care and support at the right time. Knowing early about pregnancy health also means parents can make informed decisions that may need to be taken during pregnancy.

It is recommended that mothers start antenatal care when they're around six to eight weeks pregnant. How many antenatal visits you need will depend on your individual needs, but in general most women will need between eight and ten visits before their baby is born, while others may need additional visits and tests.

What are the different types of antenatal tests?

Antenatal tests can be broadly classified as either noninvasive or invasive. Examples of non-invasive tests include the OSCAR (One-Stop Clinic for Assessment of Risk) test and NIPT (Non-Invasive Prenatal Test).

While invasive tests include amniocentesis and Chorionic Villus Sampling (CVS). These tests can identify potential major health problems that could affect the baby's quality of life, such as chromosomal abnormalities and neural tube defects.

Early screening for the mother's health in the first trimester can help ensure a healthy pregnancy and identify possible causes that could affect foetal growth. To help mother choose the most appropriate antenatal test, request an appointment with Thomson Medical.

Non-invasive antenatal tests

The OSCAR Test 

The OSCAR test is usually carried out between the 11th and 13th weeks of your pregnancy. It involves a simple blood test and a nuchal translucency (NT) ultrasound scan, which measures the thickness of the skin in the neck of your foetus and poses no risk to it.

Computer algorithm software then analyses the combined results of these tests, calculating a “risk score” with an accuracy of 85-90%. An elevated NT from a normal baseline can indicate chromosomal abnormalities, such as Down's syndrome, genetic syndromes and structural abnormalities, such as heart problems.

Non-Invasive Prenatal Test (NIPT)

If your foetus is identified as high risk during the OSCAR test, you will be referred for further tests, such as the Non-Invasive Prenatal Test (NIPT). NIPT is a simple blood test that identifies foetal cell-free DNA in the mother's bloodstream after 10 weeks of pregnancy.

The results of this antenatal test are usually available within 5 to 7 days. Like the Oscar test, NIPT does not pose a risk of harm to the foetus. This test can also be used to check whether the sex chromosomes are normal or not (detecting sex-linked chromosomal conditions), which means it can also reveal the sex of the baby.

However, it is important to remember that it is still a screening test and not a diagnostic test. A high-risk result means that the foetus has a higher chance of having a specific genetic disorder.

If the results of NIPT suggest the foetus has suspected chromosomal anomalies or a genetic disorder, a diagnostic test like chorionic villus sampling (CVS) or amniocentesis is required to confirm the diagnosis.

Pre-eclampsia screening

Pre-eclampsia is a pregnancy complication involving high blood pressure developing after 20 weeks of pregnancy. Pre-eclampsia is often associated with elevated levels of protein in the urine, biochemical abnormalities, organ damage, and foetal growth restriction.

During the first trimester, the doctor can start screening for pre-eclampsia at around 11 to 13 weeks of the gestation period to identify if you are at a high risk of developing it. This screening can be performed together with the OSCAR test.

The screening includes:

1. Maternal risk factors such as age, history of hypertension, preeclampsia, diabetes, lupus, or chronic kidney disease, and measurement of mean arterial pressure.

2. Ultrasound measurements using uterine artery Doppler ultrasound scans are used to assess blood flow in the uterine arteries.

3. Biomarkers such as PAPP-A (pregnancy-associated plasma protein A) and PLGF (placenta growth factor) are taken between 11 and 13 weeks of the gestation period in combination with the OSCAR test.

However, mothers should be aware that first-trimester screening for pre-eclampsia is not a definitive diagnosis but a useful screening test to identify pregnant women at high risk of developing pre-eclampsia before 37 weeks' gestation.

Based on the results, further diagnostic testing or close monitoring during pregnancy may be recommended to detect and treat pre-eclampsia early. Currently, the only known treatment for pre-eclampsia is delivery. Therefore, early delivery may be warranted in the case of pre-eclampsia.

If you are at high risk of pre-eclampsia, your doctor may also prescribe medication such as calcium supplements and low-dose aspirin to reduce the risk of pregnancy complications.

Invasive antenatal tests

Chorionic villus sampling (CVS)

For CVS, the doctor will take a small sample of chorionic villi, which are tiny projections that make up part of the placenta. The sample of chorionic villi can be taken through the cervix, similar to a Papanicolaou (Pap) test, or through the abdominal wall, where a needle is inserted through the abdominal wall into the placenta.

This is usually done between 10 and 12 weeks of pregnancy, much earlier than other invasive tests such as amniocentesis. This helps to detect any abnormalities earlier, giving you more time to prepare for the next steps.

CVS often causes no problems for either the mother or the foetus, although the mother may experience some soreness, spotting and leakage of amniotic fluid from the vagina. Complications from this pregnancy care, such as miscarriage, only occur in about 1 in 300 women.

Amniocentesis

Amniocentesis is often offered to pregnant women over the age of 35 because they have an increased risk of carrying a foetus with chromosomal abnormalities. However, doctors will offer it regardless of risk, and pregnant women of any age can request it, even if they are not at high risk of having a foetus with chromosomal abnormalities.

This test is usually carried out at around 15 weeks of pregnancy. A sample of the fluid surrounding your foetus (amniotic fluid) is taken for analysis by inserting a needle through your abdominal wall. This amniotic fluid contains cells that were shed by the foetus and sent to a laboratory for chromosomal analysis.

Amniocentesis allows doctors to measure the level of alpha-fetoprotein (a protein produced by the foetus), which can give a reliable indication of whether the foetus has a brain or spinal cord defect. The risks of amniocentesis are similar to those of CVS.

Percutaneous umbilical blood sampling

For percutaneous umbilical blood sampling, doctors will insert a needle through your abdominal wall and uterus into the umbilical cord to obtain a sample of your foetus's blood. This procedure is usually done when the doctors suspect that the foetus suffers from anaemia. 

If the foetus has severe anaemia, blood can be transferred to the foetus through the needle while it is still inserted in the umbilical cord. The risk of miscarriage for this test is around 1 in 500 pregnant women.

A comparison of antenatal tests

FAQ

What does antenatal testing involve?

Antenatal testing includes several tests, such as blood tests, urine tests and ultrasound scans.

Why are antenatal tests important?

Antenatal tests are an important way to check your own and your baby's health and wellbeing throughout pregnancy.

What is the main purpose of antenatal testing?

Antenatal testing is used to evaluate foetal well-being and placental function in certain pregnancies.

Are antenatal tests painful?

Most are painless, but some tests may cause discomfort or minor pain.

When should I start my antenatal checkup?

Pregnant women should start antenatal care as soon as they find out they are pregnant. The best time to start is between 4 and 8 weeks of pregnancy, no later than 12 weeks from the first day of the last menstruation.

The information provided is intended for general guidance only and should not be considered medical advice. For personalised recommendations and tailored advice, please consult a specialist by requesting an appointment with Thomson Medical today.