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Your antenatal care - Special pregnancy tests

Every pregnancy (and every pregnant woman) is different and many screening tests available in pregnancy are not routinely offered to all pregnant women either for cost reasons or because they are not thought to be useful to the majority of the population.

Having any screening test during pregnancy can be quite stressful but if you are worried about any aspect of your baby’s health, tests can also be very reassuring.

Most of these tests will check for the likelihood of a particular fetal abnormality, though  some will check for potential health problems in the mother.

Some tests will just test for the likelihood of a problem; if one of the early indicators is present, you may be offered further, more risky or invasive diagnostic tests.

For example, there are several tests to assess the risk of having a baby with the chromosomal disorder Down Syndrome, one of the most common intellectual disabilities.

Because a woman’s chance of having a baby with the disorder increases substantially with age, women over 35 are frequently offered non-invasive screening; however a large proportion of Down Syndrome births occur to women under 35 so screening is usually accessible by any pregnant woman on request.

If you have a history of any disorders or diseases in your family that may be hereditary, you may want to consider discussing these with a genetic counsellor. A directory of genetic counselling services in Australia is listed at www.genetics.com.au/services/counsel.html

Before you decide to have a test, you may want to think about potential outcomes and talk about your options, with your caregiver and perhaps with your partner.

Many antenatal clinics offer a referral to a counsellor if you feel you need help to come to a decision.

Nuchal Translucency Scan (NT)  11 to 13 weeks

One of the earliest and least invasive tests to screen for Down Syndrome is a nuchal ultrasound scan, which looks for a thickness in the back of the baby’s neck that can be an early indicator (but by no means a definitive sign) of Down Syndrome.  A blood test is often taken at 10 weeks, to test for AFP (Alpha-fetoprotein) levels, another possible Down’s indicator.

If the thickness is present, this does not mean that your baby has Down Syndrome. The results of these two tests plus your age will give your doctor a ‘risk calculation’ factor.

If the risk is higher than 1 in 300, further tests (which are more invasive and carry a small risk of miscarriage) may be offered to confirm the diagnosis.

Chorionic Villus Sampling (CVS)  10 to 14 weeks

In the Chorionic Villus Sampling (CVS) test, a specialist will use a fine needle, inserted either through the vagina or through the abdomen, to take a small number of cells from the placenta, guided by an ultrasound to ensure the needle goes to the right place.

CVS has a slightly higher risk of miscarriage than amniocentesis (around one percent for CVS versus around 0.5 percent for amniocentesis). However some women prefer it because it can be performed earlier than amniocentesis (performed at 15 weeks).

Amniocentesis  15 to 18 weeks

Amniocentesis involves the insertion of a fine needle through the abdomen, guided by an ultrasound, to take a sample of the amniotic fluid that surrounds your baby.

The test can identify over 200 different genetic disorders, however your doctor will only request testing for the disorders most likely to affect you. (This is where genetic counselling can assist.)

The test also returns information about other aspects of your baby’s health, including RH-negative status, lung maturity and various other information.

‘Short’ results can be obtained from a pathology lab within 48 hours but the full results can take two weeks.

Serum Screening or Triple Test  16 weeks

Another low-risk screen for Down Syndrome is a maternal blood test that can be taken at 16 weeks to measure the levels of three substances – oestriol, HGC (human chorionic gonadotrophin) and AFP (alpha-fetoprotein). The results of these levels in your blood, plus your age, will give your doctor a ‘risk calculation’ factor for Down Syndrome.

This test is often taken if you have missed the opportunity to have the NT test at 11 to 13 weeks. If your results indicate a higher risk, you may choose to have an amniocentesis to confirm the diagnosis.

‘Fetal Anomaly’ Scan 18-20 weeks

An obstetric ultrasound at around 18 to 20 weeks is routinely offered to most pregnant women these days. It gives medical staff the opportunity to check the probable date of the pregnancy and make sure that the baby is growing at the expected rate. It is also used to detect a number of structural abnormalities, such as hydrocephalus, dwarfism, spina bifida, cleft palate and some congenital cardiac disorders – and even multiple births, if they were not picked up earlier.

I discovered that I was expecting twins during a routine ultrasound about 20 weeks into my second pregnancy. Having experienced a normal healthy pregnancy previously, I agreed that a new student sonographer could perform the scan. She called the senior radiographer straight away – and then they broke the news.

The ultrasound can be very reassuring for most pregnant women and many fathers report that seeing their baby’s live movements through the screen really brings home the reality of their baby’s existence and can help them become better emotionally prepared for the arrival of their baby.

Obstetric ultrasound has been in use since the 1950s and is a very safe and non-invasive diagnostic procedure involving high-frequency sound waves reflecting a continuous image of the moving foetus, the placenta and surrounding tissues onto a monitor screen.  

Depending on the individual patient, a number of scans may be requested during pregnancy, from 6-7 weeks to confirm and date a pregnancy, further scans to assist with various diagnostic tests during the first and second trimester,  and possibly further scans later in pregnancy (from 32 weeks on) to check fetal growth and position. 

For more information about ultrasound:  www.ob-ultrasound.net 

Glucose Tolerance Screening  24-28 weeks

Most caregivers offer a glucose screening test between 24 and 28 weeks to identify your potential risk for gestational diabetes. This involves drinking a sugar solution provided by the test centre, then having a blood test an hour later to see how your body is processing sugar.

Only one in three of those whose blood test returns as ‘abnormal’ actually have gestational diabetes, but I you receive a positive result you will be asked to complete the more comprehensive glucose tolerance test (GTT) which involves following a specific eating plan over three days followed by a series of blood tests.

Group B Streptococcus (GBS) 35-37 weeks

Around 25 percent of women carry the group B strep bacteria in their vagina with no ill effects, however in certain cases babies exposed to GBS may develop an infection which is potentially quite serious.

Many caregivers recommend a vaginal swab to test for GBS towards the end of pregnancy so that if your labour meets certain conditions which may exacerbate risk to your baby (for example, if you have a raised temperature during labour or your waters break more than 18 hours before delivery), intravenous antibiotics may be prescribed to help prevent infection.

By Fran Molloy – journalist and mum of 4