Screening tests in pregnancy
Screening tests in pregnancy
In this article:
Routine blood tests
Non-invasive prenatal testing for Down Syndrome (NIPT)
Screening for gestational diabetes
What are screening tests and why are they done?
Screening tests are tests that try to identify the presence of a problem or risk factor, in a person who does not show obvious signs of that problem (that is, they have no symptoms, or are "asymptomatic"). This means, by definition, screening tests are applied to healthy people with no symptoms of disease or problems. Like any test in medicine, screening tests are not perfect, and the risk of an untrue result needs to weighed against the benefits of detecting the problem. In essence, this means screening tests should be used only when detecting the problem will lead to a reasonable chance of improving the health or management of the screened person.
What screening tests are done in pregnancy?
In pregnancy, several screening tests are recommended, and others are offered depending on your individual circumstances or previous history. Generally speaking, these tests look for conditions that may alter the management of your pregnancy (e.g. anaemia, Rhesus-negative blood group, the presence of chronic infections, susceptibility to other infections).
In the absence of specific conditions or complications, the Royal Australian and New Zealand College of Obstetricians and Gynaecologists recommends the following screening tests, either pre-pregnancy or when pregnancy is diagnosed:
- Full blood count: this looks for low blood ("anaemia") and may suggest the underlying cause. Sometimes further tests are required if significant anaemia is found.
- Blood group and antibody screen: this is to determine you blood type (A, B, AB, or O) and, importantly, your Rhesus status (negative or positive). The most common blood group is A-positive (type A, Rhesus-positive). About 15% of women are Rhesus negative, and this may slightly alter their pregnancy management. This test also looks for other "antibodies" which are occasionally of significance during a pregnancy.
- Rubella immunity status: most women are immune to the Rubella (or German measles) virus, thanks to vaccination of school-aged children. If you are non-immune, your doctor will talk to you about avoiding potential sources of infection during pregnancy, and you should have a vaccination shortly after baby is born (usually the next day).
- Other serological tests: these generally test for the presence of syphilis, Hepatitis B and C viruses, and HIV. Though uncommon, many women with these infections do not have risk factors in their history, and so routine screening is recommended. Additionally, recognition, prompt treatment, and/or altering practices during birth, can significantly reduce the risks to baby during pregnancy and birth.
- Urine test: this is a screening test for urinary infection, that may be asymptomatic in early pregnancy. Importantly, the presence of bacteria in the urinary system (bacteruria), when untreated, is associated with and increased risk of preterm birth owing to complicated infection of the urinary tract.
- Pap smear: if your next Pap smear would fall due during your pregnancy, you would be recommended to have a Pap smear taken at your first antenatal visit. There is no evidence that a Pap smear is harmful in pregnancy, although you should tell your doctor if you may be pregnant, as it may alter their technique for test collection.
Other screening tests that may be suggested include:
- Varicella (chickenpox) immunity: if you do not have a clear history of chickenpox, this test may be used to work out if you are immune or not. Knowing your immune status can be helpful if you are exposed to chickenpox during pregnancy. A known history of chickenpox, even as a child, is a very reliable indicator of immunity.
- Thyroid function: routine screening of thyroid function remains an area of controversy and research. However, depending on your particular personal or family history, you may be screened for thyroid disorders.
- Vitamin D: vitamin D plays an important role in bone health, and the prevalence of vitamin D deficiency appears to be increasing. Although routine vitamin D supplementation in pregnancy may be recommended, sometimes your level will be checked beforehand, especially if you are at risk of deficiency (e.g. darker-skinned women).
- Toxoplasmosis and cytomegalovirus (CMV) immunity: routine testing for evidence of previous exposure to these infections is controversial. Partly because there is no effective vaccine at present, there is no effective way of reducing transmission to a fetus from the mother, and also because a blood test demonstrating immunity does not eliminate the risk of re-infection during pregnancy. These infections are very common (usually causing a flu-like illness in children or adults) and most people are already immune. If you are at particular risk of these infection (for instance, you work in Child Care), it may be beneficial to know your immune status; however, the general rule of good hygiene and careful food preparation, and avoiding close contact with people who are ill with the "flu", is recommended regardless.
Unlike X-rays and CT scans, ultrasounds use non-ionising energy to produce clinical images, and are considered very safe in pregnancy. The following outlines some of the routine ultrasound scans most women have during their pregnancy:
- Dating or "viability" scan: this is best requested between 7 and 10 weeks after a missed period. It provides reassurance that baby is growing well, and serves to confirm your estimated due date. It is highly accurate in estimating the current duration of your pregnancy to within 4 days. If done earlier than 7 weeks, sometimes an internal ultrasound (where the probe is sheathed and placed inside your vagina) is required to see the baby.
- Nuchal translucency or "12 week" scan: this scan is done as part of Combine First Trimester Screening (CFTS) for certain chromosome disorders, such as Down Syndrome. The scan is performed between 11 and 14 weeks (usually around 12-13 weeks), and also required a blood test to complete the risk assessment. The result is not a "yes" or "no" for Down Syndrome; rather, it gives a personalised assessment of the risk of Down Syndrome in a specific woman in a specific pregnancy. A scan at this stage is also highly accurate for estimating your due date (error range of 4 days), and serves as a reassuring assessment of baby's development to date. Thus, even couples not wishing to undergo risk assessment for Down Syndrome may still have the ultrasound done, without the blood test or specific measurements required for CFTS.
- Morphology or "19 weeks" scan: during this scan a complete "anatomical survey" of baby is taken, to rule out structural development problems, such as congenital heart defects. It is usually performed between 18 and 24 weeks. Importantly, while a normal scan is very reassuring, not all structural defects can be identified during this scan. This scan also determines the position of the developing placenta, to make sure it is clear of the birth canal.
Depending on a woman's history and current pregnancy, additional ultrasounds may be required. For instance, if the placenta is thought to be low (too close to the cervix) at the morphology scan, a later scan (around 32-34 weeks) will be ordered to ensure it has moved clear. In the case of twin pregnancy, several more ultrasounds are required, every 2-4 weeks, to ensure both babies are growing well and without complications.
Other screening tests:
Gestational diabetes screen
Gestational diabetes mellitus (or GDM) is a condition that develops in pregnancy, when the body is not able to make enough insulin to keep the mother's blood sugar levels in the normal range. If untreated, it can result in complications such as blood pressure disorders and large babies, which increase the risk of difficult delivery. GDM affects around 8% of pregnancies, and usually develops in the third trimester, and so a screening test is recommended at around 26-28 weeks. If present, prompt treatment is needed to limit any risk of complications.
36-week blood test
This is a simple blood test to rule out any late pregnancy anaemia, check your platelet count, and confirm the absence of troubling antibodies, if you are Rhesus-negative.
Non-invasive prenatal testing (NIPT)
NIPT is a relatively new technology that aims to accurately predict or exclude Down Syndrome and other chromosomal problems in baby, by detecting fragments of baby's DNA in the mother's bloodstream. It simply involves a blood test from the mother, and so is "non-invasive" to the pregnancy itself. As a screening test, it is the most powerful tool for excluding Down Syndrome. Any positive result, however, still needs to be confirmed with a Diagnostic (or "invasive") test, such as amniocentesis. At the moment, there is no Medicare rebate for this test, and generally costs the patient around $400-$500. There are no Australian guidelines to guide the use of these tests, so it remains for your doctor to discuss the potential benefits of this test in your particular case. and help you decide if it is right for you.
David practices evidence-based medicine, and strives to ensure all conditions that may complicate a pregnancy are managed according to the best available medical literature and published guidelines.
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Screening tests in pregnancy
About Dr David Moore
David is a Fellow of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists, and undertook his specialist training in Queensland. He is highly skilled in the management of complex and high-risk pregnancies, and has special training in minimally-invasive surgery, endometriosis, pelvic floor and incontinence surgery. David has completed a Master of Reproductive Medicine and is skilled in the assessment and management of fertility problems, and can offer the full range of assisted reproductive treatments. He is a Senior Lecturer with The University of Queensland Medical School, and has published both medical journal and textbook contributions.