Accuracy of a blood test in early pregnancy.
Blood tests can provide a helpful estimate of how far along you are in your pregnancy, but their accuracy depends on the type of test and the context:
How Blood Tests Estimate Pregnancy Age
hCG Levels:
Blood tests measure the hormone human chorionic gonadotropin (hCG), which is produced during pregnancy.
The level of hCG in your blood rises predictably in the early weeks of pregnancy, doubling approximately every 48–72 hours in the first 8–10 weeks.
Based on the hCG level, doctors can estimate how many weeks pregnant you are, though there is a range of normal values for each week.
Accuracy:
hCG levels are a general indicator and can vary significantly between individuals.
They are most accurate for estimating early pregnancy (less than 8 weeks).
Beyond the first few weeks, ultrasounds are more reliable for dating the pregnancy.
Confirmation:
If the hCG level suggests a certain gestational age, your doctor may use an ultrasound to confirm it, as ultrasound dating (especially in the first trimester) is typically more precise