There are three sets of blood tests which are performed in pregnancy in most women. The first may have been arranged by your GP prior to your first visit at 10 weeks and will include screening of your blood for anaemia, to find out what your blood group is, whether you are immune to various viral infections, and check you do not have a urinary infection. The second is screening for anaemia, diabetes and antibodies to blood cells and this is generally done around 26-28 weeks and the last blood test is screening for anaemia again at 36 weeks.
Sometimes I will suggest additional blood tests based on your medical or previous pregnancy history or an abnormal symptom or finding during your pregnancy. It is helpful if you get your blood tests done at a Mater laboratory as then the results are available when you are in the hospital on your electronic record.
The routine Antenatal Screen includes:
- Your Blood group & Blood Group Antibodies
- Haemoglobin (to exclude anaemia)
- Rubella (German Measles) serology
- Hepatitis B & C serology
- Syphilis & HIV serology
- Random blood glucose
- A urine culture to screen for urinary infection as 5% of pregnant women will have one without symptoms
- Pap smear performed if due
Depending upon your history I might also include a check for Chicken Pox, Parvovirus, Cytomegaolvirus (CMV) and Toxoplasmosis.
If you have risk factors for diabetes we will do this early initially and then repeat it if normal later. It is usually performed at 26- 28 weeks to identify women who develop diabetes as a result of the pregnancy hormones.
Glucose Tolerance Test (GTT)
You are required to fast for 12 hours before the test, but you can have water. Most women go in the morning and have a baseline (fasting) blood sugar level taken at the start of the test. You will then be given a sweet drink and the blood test will be repeated at 1 hour and 2 hours. If the test results are abnormal then diabetes is confirmed. If early in pregnancy this may be type 2 diabetes which will still be present after the pregnancy, but if at 26-28 weeks it is likely to be gestational diabetes which will go away when your baby is born. It is a good idea to phone the laboratory the day before and check they can perform the test when you plan to go as the timing of the blood tests is important.
Blood Group Antibodies
These are checked at 28 weeks if you are rhesus negative before the anti D injection is given. Red blood cell antibodies most commonly develop in a rhesus negative women who has been exposed to rhesus positive blood. That could be from a previous birth, a miscarriage, a blood transfusion or can occur in the current pregnancy. Anti D is given to rhesus negative women who do not already have antibodies in case their baby is rhesus positive as small amounts of their baby’s blood can cross the placenta and Mum can become sensitized and make antibodies to that type of blood. Usually the baby whose blood crossed the placenta is not affected but the next one might be. Giving anti D prevents Mum developing her own antibodies as she is given enough of a dose to handle of any of baby’s stray blood cells in her own blood stream. Anti D is a blood product and it is unusual to have any reaction to it, however it is possible to have an allergic reaction to it although this is extremely rare and there is a potential risk of infection which is minimised by screening all Anti D donations before they are distributed for use.
Anti D is routinely given to Rh Neg mothers at 28 and 34 weeks in the pregnancy. If baby turns out to be rhesus negative as well no more anti D is required after birth and the two doses already given do no harm. If baby is found to be rhesus positive then a third dose is given to Mum after birth to cope with any blood cells which may have crossed the placenta in labour or at the time of birth.