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Medical Tests During Pregnancy

First Trimester Tests

Most women book with us for antenatal care during their first trimester. At your booking visit we will arrange a number of blood and other tests. We will also discuss tests that are available to screen for Down syndrome. We have an ultrasound scanner in our clinic and can confirm that your due date is correct, check for baby’s heart beat and confirm whether you are pregnant with one baby or with twins. You can read about medical tests during pregnancy in more detail by downloading our booklet on medical tests in pregnancy here.

Your booking blood tests include:



This is to check for anaemia.



This is to check your blood group and rhesus group (positive or negative). Blood group antibodies are also checked. Blood group antibodies can interfere with cross matching blood for a transfusion or cause anaemia in a developing baby.


This is to confirm that you are immune to rubella (also called german measles). Most women will have been vaccinated against rubella in childhood but a few women will have little or no immunity.


This is to check for previous infection or immunity to hepatitis B. Hepatitis B carriers can pass hepatitis onto their children. There are very effective treatments that can prevent hepatitis B being passed onto your baby after it is born. Occasionally, hepatitis B carriers will also need treatment with anti-viral drugs in pregnancy.

This screens for syphilis. This is now a rare disease but if detected treatment in pregnancy can prevent baby being infected.


HIV (the virus that causes AIDS) is still rare in pregnant women in New Zealand but carriers can infect their unborn child. Treatments are very effective in reducing the risk of fetal infection in women found to be carriers for HIV.


A urine sample can check for unexpected urine infection or the presence of bacteria in your urine that increases your risk of kidney infections later in pregnancy.


This is a measure of a woman’s blood sugar levels over the previous few weeks. High levels can indicate underlying diabetes or that you are more likely to develop diabetes later in your pregnancy. Women are more likely to develop diabetes in pregnancy if they are overweight, have polycystic ovaries or have a family history of diabetes.

Second Trimester Tests


Infections such as Chlamydia, which often cause few or no symptoms, can be checked for in pregnancy, though we usually wait until between 12 and 18 weeks to screen for this.

This is a detailed scan at 18 to 20 weeks to check baby’s brain, heart, spine and other important organs. The great majority of babies will be normal and couples find an anatomy scan very reassuring.

Occasionally, an abnormality is detected and this can have important implications for your baby’s care. For example, it may be necessary to arrange for baby to have a surgical procedure soon after birth. Some couples are very anxious that a severe abnormality will be found and a termination suggested. This is a very rare event and in most cases finding an abnormality can help paediatricians plan any treatment baby might need after it is born.

It is also important to realise that not all abnormalities in a developing baby will be detected by an ultrasound scan.

It is often possible to tell if baby is a boy or girl but this is not the main purpose of the scan. You may be offered a 4D ultrasound which will provide very detailed pictures of your baby but there is no particular medical reason to have a 4D scan.


This is done at 24 to 28 weeks of pregnancy to check if you are at an increased risk of developing gestational diabetes (diabetes in pregnancy). You will be given a sugary drink and an hour later some blood is taken. If your blood sugar is unexpectedly high you may need a further test called a Glucose Tolerance Test (GTT). For this test you will need to miss breakfast. Blood is taken to measure your “fasting” glucose levels before you are given a glucose drink and have more blood taken to measure glucose levels two hours later.

A positive polycose test is not a “panic situation” – about 10 to 15% of women will have a positive polycose test. Only about a third of women with a positive polycose test will go on to have gestational diabetes confirmed when they subsequently have a glucose tolerance test. We may advise women with an increased risk of developing gestational diabetes to have a GTT only instead of a polycose test first.

Gestational diabetes is associated with high blood sugars in pregnancy. Exposure to high sugar levels can result in baby becoming excessively large. Women who develop gestational diabetes are also at greater risk of developing diabetes in later life.


These are also checked at the same time as your polycose test to look for signs of anaemia and the development of antibodies to baby’s blood group.

Third Trimester Tests


If there is a concern that your baby's size or well-being we will organise an ultrasound scan to check that your baby is growing appropriately. Most women will not need any ultrasound scans after their 18 to 20 week anatomy scan. A few women may need several scans to monitor all is well with baby. If you go past your due date then ultrasound scans will be arranged to check that there is still plenty of liquor around baby.


Sometimes in late pregnancy it is important to check all is well with baby using a CTG machine that monitors baby’s heart beat pattern and movement. This is particularly helpful if you are still pregnant after your due date or there are worries about baby’s movements.


Most women don’t need any blood tests after their polycose test. Women who are rhesus negative or have blood group antibodies will usually have further tests at 34 to 36 weeks. Women with blood pressure problems, diabetes or other medical problems may need regular blood tests towards the end of their pregnancy.

Screening for Down Syndrome

Both ultrasound and blood tests can be used to screen for Down syndrome. At your booking visit you will be offered a “combined screening test” in which the results of a blood test and scan are combined and given as a probability or "chance" (for example 1 in 500 or 1 in 2000)that your baby has Down syndrome. For women booking for pregnancy care after 14 weeks (too late for any scan based screening) it is also possible to have a blood test alone.

There are also newer tests available that measure the levels of baby's DNA in your circulation at any time after 10 weeks (called Non-Invasive Pre-natal Testing or “NIPT”). Currently, there is no public funding for these newer tests.

Many couples find their screening options confusing. We are here to help you understand the options that are available to you.

All of us have 23 pairs of chromosomes. A child with Down syndrome has an extra twenty first chromosome (the condition is also called Trisomy 21). This is the result of a problem occurring at conception. Why it occurs is unknown. It becomes commoner with increasing maternal age. For example, at 30 years of age a woman has a one in 600 chance of carrying a baby with Down syndrome when she has screening tests at 11 to 14 weeks. By 40 years of age this chance is one in 70. Down syndrome is equally common in all ethnic groups. Children with Down syndrome have a varying degree of intellectual disability. A few will have other medical problems but the great majority of children with Down syndrome will lead fulfilling lives. For more information about Down syndrome look at the websites listed on our Useful Websites and Information page.


For women booking for antenatal care during the first trimester screening for Down syndrome consists of a blood test taken between 9 and 14 weeks of pregnancy and an ultrasound scan (called a nuchal translucency scan) at 11 to 14 weeks of pregnancy.

These screening tests start with a probability based on your age. They then calculate your personal chances based on the levels of two substances in your blood and an ultrasound measurement taken from the back of baby’s neck. The two substances in your blood (called pregnancy-associated plasma protein (PAPP-A) and beta-human chorionic gonadotrophin (Beta-hCG)) tend to occur at different levels in your blood in pregnancies where the baby has Down syndrome. The thickness of the skin at the back of baby’s neck (the nuchal translucency measurement) tends to be increased in babies that have Down syndrome. By putting these measurements together your personal chances of carrying a baby with Down syndrome is calculated. Together these tests are referred to as a first trimester "combined screening test".

Both these tests are completely safe and will tell you what your chances are that your baby has Down syndrome. They are screening tests - they will provide an accurate assessment of your chances but won’t give you a definite yes or no answer. Your results are given to you as a number or probability - for example 1 in 100 or 1 in 500. Used together as a screening test about 80 to 85 per cent of babies with Down syndrome will be picked up. It is impossible for all babies affected by Down syndrome to be picked up by screening because even for women with a low chance of having an affected baby an occasional baby will be born with Down syndrome (for example, for a chance of one in one thousand, every thousandth baby will have Down syndrome).

One purpose of the screening tests is to help women avoid amniocentesis. The chances of having a baby with Down syndrome increases with age but older women who might have considered an amniocentesis because of their age will usually still have a reassuring result from these tests.

Overall about 2% to 3% of women will opt to have an amniocentesis or chorionic villus sampling (CVS) after a first trimester combined screening test. This proportion will be slightly higher in older women because their initial chances based on their age alone will be higher.

The combined screening test will also screen for other rarer trisomies (having an extra chromosome) including trisomy 13 and 18. The nuchal translucency scan can also occasionally pick up early signs of an anatomical problem with baby.



Women booking for antenatal care after 14 weeks of pregnancy (too late for a nuchal translucency ultrasound scan) can still have a screening blood test. This is called second trimester serum screening. This can be done at up to 20 weeks of pregnancy (ideally between 14 and 18 weeks). This test also takes your chances based on your age alone and calculates a personal chance based on the levels of three different substances checked in your blood.

This later test is not quite as reliable as the combined first trimester test. There is no need to have this later test if you have already had the earlier combined test.


A newer screening test for Down syndrome is also available privately (at a cost of about $600). This is a blood test that can be done after 10 weeks. The test measures the levels of baby’s placental DNA circulating in your blood. In all pregnancies very small amounts of “cell-free fetal DNA” can be detected in maternal blood. Higher levels of DNA related to chromosome 21 will be detected in your blood if your baby has Down syndrome. This test is sometimes called “non-invasive pre-natal testing” or NIPT. NIPT will detect more than 99% of babies with Down syndrome and is much less likely to give a non-reassuring “false-positive” result than the combined screening test. It can also detect high levels of DNA associated with trisomy 18 and 13.

You may also wish to do NIPT testing as an alternative to or in addition to the combined screening test. Some women have also had NIPT testing after a non-reassuring combined screening test result. NIPT testing may reassure them enough to not have an amniocentesis. 

NIPT testing is not reliable enough to be considered a diagnostic test - not all women with a positive test will have a baby with Down syndrome. However, a negative test reduces the chances that a woman is carrying a baby with Down syndrome to well below 0.1 per cent. It is not currently part of the publicly funded national screening programme.

There are several different companies providing NIPT testing. AOC currently uses a test provided by the Victorian Clinical Genetic Services (VCGS) in Australia called PERCEPT. You can read more about the PERCEPT test by visiting the VCGS website or by downloading their patient information leaflet here.

This is a diagnostic test that will confirm if your baby does or does not have Down syndrome. It is usually done at between 15 and 17 weeks of pregnancy. A needle is passed through your abdominal wall into the fluid around baby - an ultrasound scan is used to guide the needle and make it a safe procedure for baby. A small amount of fluid is drawn off. Within this fluid are some of baby’s skins cells that can be cultured in the laboratory to check baby's chromosomes.

It can take 10 to 14 days to get a result from this test. A more rapid test (called FISH) is available that can check for Down syndrome within 24 to 48 hours is also available but at an additional cost - the doctor performing your amniocentesis can discuss this option with you.

Amniocentesis is a quick procedure taking about 10 to 15 minutes to set up and only a few minutes to do. You are welcome to bring your partner or a support person along and should plan to have a quiet 48 hours after the test. It does have a small risk of miscarriage. For every 1000 women having an amniocentesis one woman will miscarry as a result of the procedure.

This is similar to amniocentesis but it can be performed a couple of weeks earlier in pregnancy (between 11½ and 14 weeks). A very small piece of tissue is obtained from baby’s placenta using a similar needle to the one used for an amniocentesis. Technically it can be more difficult to do with a slightly higher rate of miscarriage of 1 in 500. It is usually offered to women with a particularly high risk of having a baby with a chromosomal problem.


For many couples, decision making around Down syndrome screening is straightforward and their test results are reassuring. All of us at AOC are aware that for some couples making these decisions is terribly difficult - we are very happy to talk to you by phone between appointments or arrange extra visits to clinic if you need more help in deciding what tests to have.

Down syndrome screening is optional - you do not have to have any screening if that is your preference.

We can provide you with additional written information to help guide your decision making. You can download our information leaflet on medical tests in pregnancy here. Useful websites are also listed on our Useful Websites and Information page.