Antenatal TestsA variety of standard tests are performed during pregnancy. These comprise the Antenatal Screen, Diabetes Screen and Ultrasounds.
- Blood group & Blood Group Antibodies
- Haemoglobin (to exclude anaemia)
- Rubella (German Measles) serology
- Hepatitis B & Syphilis serology
- Hepatitis & HIV serology
- Varicella (Chicken Pox) serology
- Toxoplasmosis & CMV (Cytomegalovirus) serology
- A urine culture is checked as 5% of pregnant women will have an asymptomatic infection
- Pap smear performed if necessary.
Diabetes Screen: This is performed around 28 weeks in an effort to identify women who develop diabetes due to the hormones of pregnancy.
You are not required to fast. The pathology lab will give you a sweet drink containing either 50g or 75g of glucose. 1 hour later your blood is taken and if your blood sugar level is abnormal, (> 7.8 or 8.0 respectively), you need to undergo a formal Glucose Tolerance Test (GTT)
Glucose Tolerance Test (GTT): This diagnostic test will determine if you have developed diabetes during your pregnancy. You are required to fast and a baseline blood sugar level will be taken by the pathologist at the start of the test. You will then drink a sweet drink and blood will be taken from you 1 hour and 2 hours later. Urine will also be tested for signs of glucose at these times. The whole test takes about 3 hours. If the test results are abnormal then Gestational Diabetes is confirmed.
Blood Group Antibodies: These are checked at 28 and around 36 weeks and is important for those with a NEGATIVE blood group.
Anti D is routinely given to Rh Neg mothers at 28 and 34 weeks in the pregnancy.
Optional Test: Some women will have an ultrasound at 12-13 weeks to screen for Downs Syndrome and other anomalies. A blood test is performed 3 days before the scan, with the results sent to the ultrasonographer, for use in calculating the risk of abnormality. This is a screening blood test and looks at the level of 2 hormones in the mothers blood. This result is combined with maternal age,gestational age and the ultrasound data to give a risk assessment. The blood test is not of benefit in multiple pregnancy (twins etc).
This test has replaced the Triple Test at 15 weeks as it is more reliable.
The advantage of tests like the NT and Triple test is that they are non-invasive. But, because they are occassionally wrong, we need Diagnostic tests. These are often invasive and therefore place the baby at risk. The Diagnostic test to determine if the baby truly has abnormal chromosomes is invasive.
This is performed by insertion of a needle, either into the placenta [Chorionic Villus Sample -- CVS] or into the sac of fluid around the baby [Amniocentesis]. The problem with these tests is that there is a risk of miscarriage after the procedure. That risk is about 1 in 100-150 for CVS and 1 in 250-300 for Amniocentesis. Some couples will take this small risk anyway so that they "know for sure". I refer to this as needing a YES / NO answer.
On some occasions, the result of the screening tests may suggest that the couple should consider having a diagnostic test, even though their background risk was low. This does not necessarily mean that the baby has a chromosome defect because the screening tests can be wrong. If the chromosome result comes back as normal, a careful scan will be recommended at 18 weeks. If there is nothing found on the scan, about 90 percent of these pregnancies will result in healthy live births.
AMINOCENTESIS AND CVS
An amniocentesis is an invasive test during pregnancy and is most commonly performed between 15 and 17 weeks. It can be performed anywhere between 14 weeks and delivery but is most commonly used to determine the chromosomal make up of the baby or baby's.
Under ultrasound control, a needle is passed through the abdominal wall of the mother into the uterus and into the fluid which surrounds the baby. Approximately 20 ml of fluid is removed from the uterus and sent for examination. This fluid contains cells from the baby's skin and these cells can be grown in a culture. Once a sufficient number of cells have been cultured they can then be examined and the baby's chromosome number and structure can be determined.
Fish (Flourescent In-Situ Hybridisation): Flourescent In-Situ Hybridisation is now able to give a quick numerical result within 48 hours. While not giving a complete result including structure of chromosomes, it will show if there are too many or too few chromosomes.
A complete analysis takes approximately two weeks, and should result in either 46 XX for a girl or 46 XY for a boy.
It has been generally reported that the risk of losing the baby from this test is approximately 1 in 200. A recent study of just under 4000 consecutive amniocenteses performed in Western Australia gave an overall pregnancy loss rate of 0.74 percent, which is quite favourable when compared to the calculated background normal pregnancy loss rate of 1 percent (i.e. the number of babies that would be lost between 15 and 17 weeks gestation due to other unexpected or unexplained events in pregnancy) .
Chorion Villous Sample (CVS): A CVS can be performed between 10 -12 weeks and is a biopsy of the placenta. It has twice the miscarriage rate of amniocentesis but because it can be performed earlier, the options available to the parents are greater.
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