Blood Tests During Pregnancy: What Each Test Is For
From the first prenatal visit to the third trimester, pregnant women receive a battery of blood tests. Here is what each one screens for, when it is done, and what a result outside the normal range means for you and your baby.
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Get My Score →First Trimester Blood Tests (Weeks 1–12)
| Test | Purpose | Normal / Target |
|---|---|---|
| Blood type and Rh factor | Rh-negative mothers need anti-D immunoglobulin to prevent Rh sensitisation | Rh-negative confirmed before 28 weeks |
| Full blood count (CBC) | Screen for anaemia — maternal iron needs double in pregnancy | Haemoglobin above 11 g/dL; MCV normal |
| Rubella immunity | Non-immune mothers cannot be vaccinated in pregnancy — risk to baby if exposed | IgG positive = immune |
| Syphilis (RPR/VDRL) | Congenital syphilis causes fetal loss and neonatal disability — mandatory screen | Negative |
| Hepatitis B surface antigen | Perinatal transmission risk; newborn immunoprophylaxis if positive | Negative |
| HIV | Treatment in pregnancy prevents vertical transmission to nearly zero | Negative; if positive: immediate treatment |
| hCG + PAPP-A (combined screening) | First-trimester chromosome aneuploidy screen (combined with NT ultrasound) | Interpretation varies by laboratory algorithm |
| TSH | Thyroid dysfunction common in pregnancy; hypothyroidism in pregnancy impairs fetal brain development | First trimester TSH target below 2.5 mIU/L |
Second Trimester Tests (Weeks 13–26)
The glucose challenge test (GCT) at 24–28 weeks screens for gestational diabetes — a 50g glucose load followed by a 1-hour blood glucose. A result above 130–140 mg/dL requires confirmatory 3-hour glucose tolerance test (GTT). Gestational diabetes affects 7–10% of pregnancies and, if uncontrolled, raises the risk of macrosomia, shoulder dystocia, neonatal hypoglycaemia, and increased lifetime diabetes risk for both mother and child. Quadruple screen (AFP, hCG, estriol, inhibin A) at 15–20 weeks supplements first-trimester chromosome screening and screens for neural tube defects via elevated AFP.
Required Blood Tests at Each Antenatal Stage
Third Trimester Tests (Weeks 27–40)
Haemoglobin recheck at 28 weeks identifies iron-deficiency anaemia, which peaks in the third trimester as fetal iron demands are highest. Group B Streptococcus (GBS) swab at 35–37 weeks (rectal/vaginal, not blood) determines if intrapartum antibiotic prophylaxis is needed. Repeat CBC and antibody screen if Rh-negative. In high-risk pregnancies, additional tests may include sFlt-1:PlGF ratio to predict pre-eclampsia risk, liver enzymes (for HELLP syndrome screening), and repeat glucose if gestational diabetes was borderline.
Nutrition Deficiencies That Require Blood Monitoring in Pregnancy
Ferritin and Iron: The Most Undertreated Deficiency in Pregnancy
Iron requirements roughly double in pregnancy. The WHO defines anaemia in pregnancy as haemoglobin below 11 g/dL — but ferritin below 30 ng/mL indicates depleted iron stores even before haemoglobin falls, and is associated with adverse pregnancy outcomes including preterm birth and postpartum depression. Routine prenatal iron supplementation is standard, but women with prior iron deficiency, vegetarian diets, or multiple pregnancies may need earlier and higher-dose iron supplementation guided by ferritin levels.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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