Understanding Blood Tests During Pregnancy - A Week-by-Week Guide

Pregnancy involves a series of important blood tests at different stages to monitor both maternal and foetal health. Understanding what each test checks for — and when to take it — helps expecting parents stay informed and prepared.

First Trimester Blood Tests (Weeks 1–12)

• Blood Type and Rh Factor: Determines your blood group and whether you are Rh-positive or Rh-negative. If Rh-negative, you may need Rh immunoglobulin injections to prevent complications.
• Complete Blood Count (CBC): Checks for anaemia, infection, and platelet levels.
• Rubella Immunity: Confirms whether you are immune to rubella (German measles), which can cause serious birth defects if contracted during pregnancy.
• Hepatitis B and HIV Screening: Both can be transmitted to the baby during birth; early detection allows for preventive treatment.
• Thyroid Function (TSH): Thyroid disorders can affect foetal brain development; early detection is critical.
• NIPT (Non-Invasive Prenatal Testing): From week 10 onwards, NIPT screens for chromosomal conditions like Down syndrome (trisomy 21), trisomy 18, and trisomy 13 using a simple maternal blood draw.

Second Trimester Blood Tests (Weeks 13–26)

• Quad Screen / Maternal Serum Screening: Measures four markers (AFP, hCG, estriol, inhibin A) to assess risk of Down syndrome, trisomy 18, and neural tube defects.
• Glucose Challenge Test (GCT): Screens for gestational diabetes, typically done between weeks 24–28.
• Glucose Tolerance Test (GTT): A follow-up test if the GCT result is elevated, to confirm or rule out gestational diabetes.
• Anaemia Recheck: Iron-deficiency anaemia is common in the second trimester as blood volume increases.

Third Trimester Blood Tests (Weeks 27–40)

• Group B Streptococcus (GBS) Screening: Done between weeks 35–37. GBS can be passed to the baby during delivery; if positive, antibiotics are given during labour.
• Repeat CBC and Iron Levels: To check for anaemia as delivery approaches.
• Repeat HIV and Hepatitis B: Some guidelines recommend repeat screening in the third trimester for high-risk pregnancies.

Genetic Testing During Pregnancy

Beyond routine blood tests, genetic testing options during pregnancy include:

• NIPT: Non-invasive screening for chromosomal abnormalities from week 10.
• Chorionic Villus Sampling (CVS): Diagnostic test done between weeks 10–13 to detect chromosomal and genetic disorders.
• Amniocentesis: Diagnostic test done between weeks 15–20 to detect chromosomal abnormalities and genetic conditions.
• Carrier Screening: Tests both parents for recessive genetic conditions that could be passed to the baby.

Conclusion

Staying on top of your pregnancy blood tests is one of the most important things you can do for your health and your baby’s health. Work closely with your obstetrician to ensure all recommended tests are completed at the right time.