So many questions and worries during pregnancy - pathology has the answers

Blood tests during pregnancy – what to expect when you’re expecting

During pregnancy, it is common for women to experience a range of emotions and expectant mothers may feel overwhelmed by the many areas of antenatal care recommended.

Some aspects of your antenatal care involve medical tests to monitor the health of you and your baby. The following guide outlines the different tests available throughout each trimester, and the purpose of each test. Of course, every test is optional.

Trimester 1

At 6-8 weeks’ pregnancy

When you first visit your GP to confirm your pregnancy, you will be offered several blood tests. A full blood examination (FBE) will check for possible iron deficiency and thalassaemia risk. A ferritin level may be included and is more sensitive than the full blood alone to determine reduced iron stores, to make sure you’re not at risk of anaemia, which can make you tired and weak. A blood group test will determine not just the ABO group but more importantly identify individuals with a Rhesus negative blood type who should be offered therapy during pregnancy to prevent haemolytic disease of the fetus and new-born, which can lead to pregnancy loss and new-born jaundice.

You’ll also be screened for infectious disease exposure or immunity including HIV, and Hepatitis B and C which can put babies at risk of future liver disease if not treated. Testing for antibodies to rubella and chicken pox is conducted as exposure to these viruses during pregnancy can cause birth defects if you’re not immune. You can also talk to your doctor about whether testing for chlamydia, syphilis, thyroid disease and vitamin D is necessary.

After 10 weeks’ pregnancy

You will be offered the nuchal translucency test between the 10th and 14th week of the pregnancy to determine the chance of the baby being affected by certain chromosomal abnormalities such as Down syndrome, Edwards syndrome or Patau syndrome.

This combines information obtained by an ultrasound and blood tests.  The ultrasound is performed predominantly to assess nuchal translucency, the thickness of the fold at the back of the baby’s neck. Together with the maternal age, weight, gestation and blood test results, the chance of a baby affected by these abnormalities is determined. The test cannot accurately diagnose a fetus with a chromosomal condition.

Additional blood tests are also available for expectant parents with close blood relatives who carry other genetic disorders.

Trimester 2

Between 15 and 17 weeks of pregnancy
Multiple Marker Screening or Maternal Serum Screening involves a blood test that looks for Down syndrome, Edwards syndrome and neural tube defects such as spina bifida and anencephaly, in which the skull does not form properly. The screen looks for four chemicals in the blood: alpha-feto protein, unconjugated estriol, free beta hCG and dimeric inhibin A.

These results are combined with the results from the nuchal translucency test as well as other information about the pregnancy to assess the risk of a fetus having abnormalities. All tests have limitations and may not be diagnostic alone.

The full blood examination is usually repeated towards the end of the second trimester, looking for the development of iron deficiency anaemia, which is extremely high in pregnancy, as well as to check the platelet count which may sometimes fall as a result of pregnancy complications.

Non-invasive prenatal testing

Non-invasive prenatal testing (NIPT) can offer better accuracy in detecting chromosomal abnormalities. This is a blood test that tests fetal DNA released into the mother’s blood. Testing is recommended between 11 and 16 weeks and may be carried out after a woman has had the nuchal translucency test or following the results of other blood tests.

Tests such as amniocentesis or chorionic villus sampling (CVS) are also available. These are more invasive tests; amniocentesis tests fetal cells in the amniotic fluid and CVS tests cells taken directly from the placenta. Because of their invasive nature these tests carry a risk of miscarriage making NIPT a safer option.

Trimester 3: Between 24 – 28 weeks

Between three and eight per cent of women will get gestational diabetes between the 24th and the 28th week of pregnancy, sometimes earlier. It usually goes away after the baby is born. A blood glucose tolerance test screens for gestational diabetes. The test uses three blood samples: the first sample is taken before a standardised glucose drink is consumed, the second sample one hour after and the third sample two hours after the glucose load. A pathology laboratory compares results from all samples to see if they indicate gestational diabetes. The condition can be managed with healthy eating, physical activity, monitoring your blood glucose levels and sometimes medication.

Rhesus negative women will also have their antibody status checked prior to receiving their first dose of anti-D at 28 weeks.

Other tests

Pregnant women may also be tested for Group B Streptococci (GBS) bacteria via a vaginal or anorectal swab at 35-37 weeks. These bacteria occur naturally in some women and are usually not harmful, however if passed on to a newborn in the birth canal, the baby can become very ill. Pregnant women carrying these bacteria can be offered antibiotic treatment during labour as a measure to help protect a baby from becoming infected.

Finally, women may also be offered pathology tests if they fall ill while pregnant or if they are in a high-risk group – particularly during the early stages of pregnancy. Certain conditions can affect the unborn baby such as cytomegalovirus (CMV), toxoplasmosis and herpes simplex virus (HSV) and pathology can diagnose these conditions so that steps can be taken to protect mother and baby.

References:

https://www.vcgs.org.au/tests/maternal-serum-screening

https://www.thewomens.org.au/health-information/pregnancy-and-birth/pregnancy-problems/pregnancy-problems-in-later-pregnancy/gestational-diabetes/