What Is a Blood Pregnancy Test and How Does It Work?

A Quantitative (hCG) or Blood Pregnancy Test Tells a Great Deal

doctor drawing woman's blood
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You may have taken a home pregnancy test before, but now your doctor may be suggesting that you also take a blood test — But what exactly is a blood pregnancy test and how it differs from a urine test?

What Is a Blood Pregnancy Test and How Does It Work?

A blood pregnancy test is a quantitative hCG blood test to show how much hCG (or human chorionic gonadotropin) is in your blood. This is the type of blood pregnancy test that most people are referring to when they talk about blood tests for pregnancy.

To test your blood for hCG, a blood sample is taken from one of your veins, through a procedure called a venipuncture.

HCG is a hormone secreted in pregnancy and detectable by blood test about day 8 to 10 after conception. Whether you are taking a home pregnancy test that uses urine or a blood pregnancy test from your doctor or midwife, the results will be based on measurements of this hormone – be it in your urine or blood.

However, a blood test is more sensitive and can offer more information than a home urine test. You may have a qualitative hCG test to measures hCG in your blood. The results are very clear: Yes, you are pregnant because we found hCG or, no, you are not pregnant because we didn't find it.

Results from quantitative tests give your doctor or midwife not just a yes/no result. They provide measurements that can be compared. In general, your hCG will nearly double about every two days in early pregnancy. So by having multiple blood tests about 48 hours apart, you can track this hCG number and get a better read on the pregnancy. These serial blood tests can help your practitioner monitor your pregnancy for miscarriage or ectopic pregnancy as well as the possibility that you’re carrying multiples.

Due to stress, expense, and other factors, these tests are not done routinely for every pregnant woman. Talk to your doctor or midwife if you think that a blood test for pregnancy is right for you. Otherwise, you should be able to rely on the results of your home pregnancy tests (HPT).

How the Results Are Interpreted

Normal results from a quantitative hCG test or blood pregnancy test would mean that hCG levels are rising rapidly during the first trimester of pregnancy and then slightly declining. Abnormal results, on the other hand, can mean a variety of things, where higher than normal level may indicate:

  • More than one fetus, i.e. twins or triplets
  • Infection or malignant tumors of the uterus
  • Non-cancerous tumors of the uterus
  • Ovarian cancer
  • Testicular cancer (in men)
  • Sometimes just an indication of a normal pregnancy

While lower than normal levels may mean

  • A possible death of the fetus
  • An incomplete or complete miscarriage
  • An ectopic pregnancy

Risks of Having Blood Drawn

There is very little risk associated with a blood pregnancy test. Aside from some bruising a the site of the injected needle, most people do not have any other side effects from having blood drawn, however, there are certain individuals where excessive bleeding, fainting or infection may be a possible risk from having blood drawn. Let your doctor know if you have ever had an issue with having blood drawn before. 

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Article Sources

  • Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.