Blood Tests During Pregnancy
A trimester-by-trimester guide to the blood tests you will have during pregnancy in Australia — what each one screens for, when it happens, and what the results mean.
Why So Many Blood Tests During Pregnancy?
Pregnancy places extraordinary demands on your body. Your blood volume increases by 50%, your immune system is deliberately suppressed to prevent rejection of the baby, and your metabolism shifts to prioritise nutrient delivery to the placenta. These changes create specific vulnerabilities that routine blood tests are designed to catch early.
In Australia, most pregnancy blood tests are fully bulk billed under Medicare when ordered by your GP, midwife, or obstetrician. The booking bloods at your first antenatal visit are the most comprehensive — typically 6-8 tests from a single blood draw. Additional tests are spaced across the pregnancy to catch conditions that develop later, like gestational diabetes and Group B Strep.
Understanding what each test checks for can help reduce anxiety about results and ensure you ask the right questions at your antenatal appointments. This guide covers the standard tests recommended by RANZCOG (Royal Australian and New Zealand College of Obstetricians and Gynaecologists).
Essential Pregnancy Blood Tests Explained
Blood Group, Rh Factor, and Antibodies
Why this is tested: Your blood group and Rh factor are among the very first tests at your booking appointment (around 8-12 weeks). If you are Rh-negative (about 15% of Australians) and your baby is Rh-positive (inherited from the father), your immune system may produce antibodies against the baby's red blood cells. This is called Rh isoimmunisation and can cause haemolytic disease of the newborn (HDN) — a condition where the baby's red cells are destroyed, leading to severe anaemia, jaundice, and in serious cases, brain damage or stillbirth. The antibody screen also checks for other irregular antibodies that could affect the pregnancy.
What to expect: No specific "optimal range" — this is a classification test. Rh-negative mothers receive anti-D immunoglobulin injections at 28 and 34 weeks to prevent sensitisation. If the antibody screen is positive, further quantification and monitoring is required throughout the pregnancy.
Good to know: Even if you had blood group testing years ago, it must be repeated each pregnancy because the antibody screen checks for NEW antibodies that may have developed since your last pregnancy or blood transfusion. The test is repeated at 28 weeks to check for antibodies that developed during the pregnancy itself.
Full Blood Count (Anaemia Screening)
Why this is tested: Anaemia is the most common blood disorder in pregnancy, affecting 15-20% of Australian pregnant women. Your blood volume increases by 30-50% during pregnancy to supply the placenta and growing baby, but red blood cell production often cannot keep up — leading to a relative drop in haemoglobin called physiological dilutional anaemia. True iron deficiency anaemia on top of this physiological change can cause fatigue, breathlessness, increased infection risk, and is associated with preterm birth, low birth weight, and postpartum haemorrhage. Platelets are also monitored because gestational thrombocytopenia affects 5-10% of pregnancies.
What to expect: Haemoglobin above 110 g/L in the first trimester, above 105 g/L in the second and third trimesters. Below 100 g/L requires active treatment (oral or IV iron). MCV below 80 fL suggests iron deficiency; above 100 fL suggests B12 or folate deficiency. Platelets above 150 x10^9/L is normal; below 100 requires investigation.
Good to know: Haemoglobin naturally drops in the second trimester due to plasma volume expansion — this is normal and does not always mean you are anaemic. Your midwife or obstetrician will interpret your levels in context. However, do not dismiss genuinely low levels as "just pregnancy" — true anaemia needs treatment.
Iron Studies (Ferritin)
Why this is tested: Your iron requirements increase dramatically during pregnancy — from 18 mg/day pre-pregnancy to 27 mg/day, peaking at 40+ mg/day in the third trimester. The baby draws on your iron stores to build its own blood supply and creates an iron reserve that lasts for the first 6 months of life. Ferritin (your iron storage protein) is the most reliable marker in early pregnancy. Depleted iron stores increase the risk of preterm delivery by 2-3 times and double the risk of postpartum depression. The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) recommends checking ferritin at the booking visit and again at 28 weeks.
What to expect: Ferritin above 30 ug/L in early pregnancy. Below 30 ug/L indicates depleted stores even if haemoglobin is normal — oral iron supplementation should start. Below 15 ug/L is severe depletion and may require IV iron infusion (Ferinject), which is the fastest way to replenish stores. IV iron is commonly administered from the second trimester onward and is Medicare-rebated.
Good to know: Ferritin rises in inflammation, infection, and pre-eclampsia — all of which can occur in pregnancy. A "normal" ferritin during a pregnancy complicated by inflammation may actually mask iron deficiency. If CRP is also elevated, interpret ferritin with caution. Some obstetricians use a higher cutoff of 50 ug/L during pregnancy to account for this.
Glucose Tolerance Test (Gestational Diabetes)
Why this is tested: Gestational diabetes mellitus (GDM) affects 12-14% of Australian pregnancies — one of the highest rates in the developed world. The placenta produces hormones (human placental lactogen, cortisol, progesterone) that create insulin resistance, ensuring the baby gets enough glucose. In some women, the pancreas cannot produce enough insulin to overcome this resistance, resulting in GDM. Uncontrolled GDM increases the risk of large babies (macrosomia), birth injuries, neonatal hypoglycaemia, and pre-eclampsia. Women with GDM also have a 50% lifetime risk of developing type 2 diabetes. The oral glucose tolerance test (OGTT) at 24-28 weeks is the gold standard screening test.
What to expect: OGTT diagnostic thresholds (ADIPS criteria used in Australia): Fasting glucose below 5.1 mmol/L, 1-hour below 10.0 mmol/L, 2-hour below 8.5 mmol/L. ONE abnormal value is sufficient for GDM diagnosis. HbA1c is checked at the booking visit to screen for pre-existing diabetes (above 48 mmol/mol = diabetes, 42-47 = high risk requiring early OGTT).
Good to know: You must fast for 8-10 hours before the OGTT and sit quietly during the 2-hour test — physical activity lowers glucose and can mask GDM. The glucose drink contains 75g of glucose (equivalent to drinking nearly 2 litres of soft drink). Nausea is common; if you vomit, the test must be repeated on another day. Some women with risk factors (BMI above 30, family history, previous GDM, PCOS) should have an early OGTT at 16-18 weeks in addition to the routine 24-28 week test.
Thyroid Function (TSH)
Why this is tested: Thyroid disorders affect 2-5% of pregnancies and can have profound effects on both mother and baby. In the first trimester, the baby is entirely dependent on maternal thyroid hormones for brain development — the baby's own thyroid does not start functioning until 12-14 weeks. Untreated hypothyroidism increases the risk of miscarriage by 4 times, pre-eclampsia by 3 times, and is associated with reduced IQ in the child. Hyperthyroidism (usually from Graves' disease) increases the risk of preterm birth, low birth weight, and thyroid storm. Pregnancy-specific TSH reference ranges are lower than standard ranges because hCG (pregnancy hormone) stimulates the thyroid.
What to expect: TSH in the first trimester should be 0.1-2.5 mIU/L (lower than the standard range of 0.5-4.0). Second trimester: 0.2-3.0 mIU/L. Third trimester: 0.3-3.0 mIU/L. Free T4 should be 12-22 pmol/L. Anti-TPO antibodies are checked if TSH is abnormal — positive antibodies indicate Hashimoto thyroiditis and increase miscarriage risk even when TSH is borderline.
Good to know: Standard (non-pregnant) TSH reference ranges should NOT be used during pregnancy. A TSH of 3.5 mIU/L is "normal" outside pregnancy but is too high in the first trimester and would warrant treatment with levothyroxine. Ensure your GP or obstetrician uses trimester-specific reference ranges. Also: hCG can suppress TSH in early pregnancy (especially with hyperemesis gravidarum), mimicking hyperthyroidism — this usually resolves by 14-16 weeks.
Infectious Disease Screening
Why this is tested: Infectious disease screening is done at the booking visit to identify infections that can be transmitted to the baby during pregnancy or birth. Rubella (German measles) infection in the first trimester causes congenital rubella syndrome — deafness, cataracts, heart defects, and intellectual disability. The rubella IgG test checks whether you are immune (from vaccination or past infection). Hepatitis B can be transmitted during birth, but giving the baby immunoglobulin and vaccine within 12 hours of birth prevents infection in 95% of cases. HIV transmission can be reduced from 25% to less than 1% with antiretroviral treatment during pregnancy. Syphilis has resurged in Australia (cases tripled 2014-2024) and causes devastating congenital syphilis if untreated.
What to expect: Rubella IgG above 10 IU/mL indicates immunity. Hepatitis B surface antigen (HBsAg), Hepatitis C antibody, HIV 1&2 antibody, and syphilis serology (RPR/VDRL) should all be negative. A positive result for any triggers a specific management pathway — early detection allows treatment that dramatically reduces transmission risk.
Good to know: Many younger Australians have waning rubella immunity despite childhood vaccination. If your rubella IgG is low (below 10 IU/mL), you are NOT immune and should be vaccinated AFTER delivery (the vaccine cannot be given during pregnancy as it is a live vaccine). Avoid contact with anyone who has rubella during the pregnancy. Syphilis screening is now recommended at booking AND at 28 weeks due to the ongoing outbreak in Australia.
Vitamin D
Why this is tested: Vitamin D deficiency during pregnancy is associated with pre-eclampsia, gestational diabetes, preterm birth, and low birth weight. The baby accumulates vitamin D stores during the third trimester for use in early infancy — a deficient mother produces a deficient baby. Neonatal vitamin D deficiency causes rickets (soft, deformed bones) and has been linked to increased risk of respiratory infections, eczema, and food allergies in the first year of life. RANZCOG recommends vitamin D screening in pregnancy, particularly for women with darker skin, limited sun exposure, or those who wear covering clothing.
What to expect: Vitamin D should be above 50 nmol/L (adequate), ideally above 75 nmol/L during pregnancy. Below 50 nmol/L requires supplementation with 1000-2000 IU daily. Below 30 nmol/L is severe deficiency requiring a loading dose. RANZCOG recommends routine supplementation of 400-600 IU/day for all pregnant women, with higher doses for those who are deficient.
Good to know: Vitamin D testing is bulk billed during pregnancy when your GP documents the clinical indication. Standard prenatal vitamins contain only 200-400 IU of vitamin D, which is insufficient to correct a deficiency. If your level is below 50 nmol/L, you will need a separate vitamin D supplement in addition to your prenatal vitamin.
Group B Streptococcus (GBS) Screening
Why this is tested: Group B Streptococcus (GBS) is a bacterium that colonises the vagina and rectum of 15-25% of Australian women without causing any symptoms. During vaginal birth, the baby can be exposed to GBS in the birth canal. Early-onset GBS disease in newborns (within 7 days of birth) causes sepsis, pneumonia, and meningitis — affecting approximately 0.5-1 per 1000 live births and carrying a 5-10% mortality rate. GBS screening is performed at 35-37 weeks gestation via a vaginal and rectal swab (not a blood test). If positive, intravenous antibiotics (usually penicillin) are given during labour to reduce transmission by over 80%.
What to expect: This is a positive/negative result. If positive, intrapartum antibiotics are recommended. The swab must be done at 35-37 weeks because GBS colonisation can be intermittent — a negative result earlier in pregnancy does not guarantee negative status at birth.
Good to know: GBS screening is a swab, not a blood test — but it is included in this guide because it is one of the most important late-pregnancy screening tests and is often discussed alongside blood tests. Some hospitals offer universal screening, while others use a risk-based approach. Ask your midwife or obstetrician about your hospital's policy. Women with GBS bacteriuria (GBS in a urine culture) at any point during pregnancy are automatically considered positive and do not need the 35-37 week swab.
Pregnancy Testing Timeline
This table shows when each test is typically performed and whether it is bulk billed under Medicare. Your care provider may adjust the schedule based on your individual risk factors.
| Test | When | Screens For | Medicare Status |
|---|---|---|---|
| Blood Group and Antibodies | 1st (8-12 weeks) | Rh factor, irregular antibodies | Bulk billed |
| Full Blood Count | 1st + 3rd (28 weeks) | Anaemia, platelet disorders | Bulk billed |
| Iron Studies (Ferritin) | 1st + 3rd (28 weeks) | Iron deficiency | Bulk billed |
| Rubella IgG | 1st (booking) | Rubella immunity | Bulk billed |
| Hepatitis B, C, HIV, Syphilis | 1st (+ syphilis at 28w) | Transmissible infections | Bulk billed |
| Thyroid Function (TSH) | 1st (if indicated) | Hypo/hyperthyroidism | Bulk billed |
| HbA1c | 1st (booking) | Pre-existing diabetes | Bulk billed |
| Vitamin D | 1st (booking) | Deficiency risk | Bulk billed* |
| Glucose Tolerance Test (OGTT) | 2nd (24-28 weeks) | Gestational diabetes | Bulk billed |
| Antibody Screen (repeat) | 3rd (28 weeks) | New antibodies in Rh-negative women | Bulk billed |
| Group B Strep Swab | 3rd (35-37 weeks) | GBS colonisation | Bulk billed |
* Vitamin D is bulk billed with a clinical indication (pregnancy is a valid indication at most pathology providers). Additional tests like thyroid antibodies, coeliac screen, or TORCH screen may be ordered if clinically indicated — these are also typically bulk billed with appropriate documentation.
Questions to Ask at Your Booking Appointment
Am I Rh-negative? Will I need anti-D injections?
Is my iron level adequate for pregnancy, or should I start supplements?
Am I immune to rubella?
Should I have thyroid function tested? (especially if family history)
When will my glucose tolerance test be scheduled?
Is my vitamin D level adequate?
Do I have any risk factors for early gestational diabetes screening?
What is your hospital's policy on Group B Strep screening?
Warning Signs That Need Immediate Attention
While most pregnancy blood test abnormalities are manageable, certain symptoms during pregnancy require urgent assessment. Do not wait for your next appointment — call your maternity unit or go to emergency.
Related Reading
Track Your Pregnancy Blood Tests
Upload your pregnancy blood test results to SmarterBlood. Our AI will track your iron, haemoglobin, thyroid, and glucose levels across trimesters — showing you trends and flagging anything that needs attention. Free and private.
SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your midwife, obstetrician, or GP with questions about your pregnancy care.