How Soon Can a Blood Test Detect Pregnancy?
|Expert Reviewed||Dr. Mark Trolice, Director of Fertility Care|
How Early Can a Blood Test Detect Pregnancy Hormones?
"Pregnancy blood tests are reliably able to detect pregnancy at much lower ranges of circulating pregnancy hormone in the blood and thus can catch a pregnancy in its very early stages," says Dr. Cindy M. Duke, a physician with the Nevada Fertility Institute in Las Vegas. "Circulating levels of pregnancy hormone would have to be much higher in order for it to be detected in the urine (home pregnancy test)."
Do you think you might be pregnant, but you are not sure about the reliability of the home test you've taken? Or perhaps you want to know now if you're pregnant but it's too early to take a urine test, so you are considering a blood test.
When can you trust the home test and when do you need to take a blood test instead?
There is much to consider when deciding whether or not to get the blood test. How accurate are they? How much trouble and cost are they? How soon can you take one and when will you get the results?
What Is an hCG Blood Pregnancy Test?
There are two types of blood tests to determine pregnancy:
- Qualitative hCG blood serum tests check to see if a hormone called human chorionic gonadotropin is present in your blood. HCG is a hormone produced by the body during pregnancy. A positive result means you are pregnant and a negative result means you are not pregnant.
- Quantitative hCG blood serum tests also check to see if hCG is present in your blood, but these tests can detect even trace amounts of the hormone. Because it is more precise, this test helps to determine whether the pregnancy is normal and healthy.
How Early Can a Blood Test Tell Me I'm Pregnant?
"The earliest a blood test can detect hCG is approximately 8-10 days post-ovulation," says Dr. Mark Trolice, Director of Fertility Care at the IVF Center in Winter Park, Florida. "What makes things more challenging? A woman who has irregular menstrual periods and does not know when she ovulated."
Both qualitative and quantitative hCG blood serum tests can accurately detect pregnancy 8-10 days after ovulation (about a week before your period is due). This is an advantage to taking the blood test: You can take it sooner than the urine test.
How Accurate Are Blood Pregnancy Tests?
The blood test for hCG is the most accurate to detect a pregnancy, says Dr. Mark Trolice.
Blood tests have a 99% accuracy rate, so they are almost always accurate. The main reason for this is that the quantitative test can detect even the smallest traces of the pregnancy hormone hCG. However, hormone levels can vary from one woman to another.
"The range of normal beta hCG levels is very wide," says Dr. Serena Chen, a fertility expert at the Institute for Reproductive Medicine and Science at Saint Barnabas Medical Center in Livingston, New Jersey, "so it's important to have the help of your doctor to interpret test results."
How Long Do You Have to Wait for the Results of a Blood Test?
So you've gone to the doctor's office, waited to be seen, and had your blood drawn. Of course, it's not over.
Your blood has to be sent to a lab to be analyzed. How long it takes for them to give you the results varies from lab to lab. It could take an hour or several days. A couple of days is a typical waiting period for lab results.
How Accurate Are Urine Pregnancy Tests?
Comparatively, urine tests, or home tests, can be inaccurate 6% of the time. They are more accurate when taken no earlier than seven days after a missed period. Taking a home pregnancy test too soon can produce an inaccurate result.
Have you tried a home pregnancy test or a blood test?
Pregnancy Blood vs. Urine Tests
- are more expensive than home urine tests
- have to be given at a doctor's office
- can be done just one week after ovulation
- can take up to 48 hours to produce a result
- are more accurate than urine tests.
- are convenient and private
- are less expensive
- produces results in minutes
- are only accurate when done seven days after a missed period
- produce more false positive results than blood tests.
Using Blood Tests to Diagnose Ectopic or Tubal Pregnancy
"An early ectopic pregnancy may yield a false negative if you test too early," says Dr. Chen. If you have concerns about the possibility of ectopic pregnancy, discuss them with your doctor.
Ectopic pregnancies are fertilized eggs that have implanted in the fallopian tube instead of the uterus. This type of pregnancy can present similar symptoms as a normal pregnancy but can be life-threatening for the woman.
A quantitative hCG blood test can help diagnose (or rule out) an ectopic pregnancy. They are more precise than qualitative tests and detect lower levels of hCG in the blood. In an ectopic pregnancy, hCG levels are usually lower than in a normally developing pregnancy and also rises at a slower-than-normal rate.
While an ultrasound exam may be necessary to confirm the diagnosis, the blood test can be done earlier, which is important given the risks of this type of pregnancy.
False Positive Results of Blood Pregnancy Test
False positive results of pregnancy blood tests are rare, but they do happen. Some conditions can lead to a result of pregnancy when the woman is, in fact, not pregnant.
Because blood tests detect the presence of the hormone hCG, any level of hCG—even if it is there for another reason—could lead to a false positive test result. Some non-pregnancy reasons for hCG to be in the blood are:
- Biochemical pregnancy, or a pregnancy that is lost before the woman even knew she was pregnant.
- Pituitary hCG, or hormones created by the pituitary gland that are similar to hCG. It is possible for women to have low levels of hCG in the blood and not be pregnant.
- Antibodies that interfere with hCG tests and lead to elevated hormone levels.
What Is Phantom hCG?
Phantom hCG is a false positive pregnancy blood test.
In these cases, the blood test for hCG cross-reacts with non-pregnancy related proteins in the blood called heterophile antibodies.
"When this occurs, it can actually have serious consequences unless your physician is aware of this false positive risk," says Dr. Mark Trolice. "What are the risks? Ectopic pregnancy, where an embryo implants outside the uterus. This is a potentially life-threatening event but is often conservatively treated using an injection of the chemotherapy agent, methotrexate."
But if the blood test for hCG is a false positive, and there is, in fact, no ectopic pregnancy, the woman may be needlessly exposed to repeated injections of methotrexate with risks of serious side effects from this medicine.
Another condition that can lead to a false positive, says Dr. Trolice, is gestation trophoblastic disease, or cancer of pregnancy tissue. In this scenario, persistent low levels of phantom hCG can be misinterpreted for this disease resulting in aggressive but unnecessary medical intervention.
How to avoid over-treating a false positive hCG test? The urine pregnancy test, because heterophile antibodies are not present in urine.
Can Blood Pregnancy Tests Give False Negative Results?
If you have had a pregnancy blood test and it came back negative, but you feel pregnant, it is possible that you really aren't pregnant. Many of the symptoms of an approaching period (nausea, fatigue, crampiness, breast tenderness) can be confused with pregnancy.
Is It Possible to Take a Pregnancy Blood Test Too Early?
Yes. In rare cases, it takes longer than usual for hCG hormone levels to build up in the blood. If you had a negative blood test result but you really feel pregnant, wait a few days and get tested again.
So as you can see, there are many things to consider when you are trying to figure out whether to take the blood test and what to expect from it. It is more expensive and less convenient. You will have to wait for the lab to get results. However, it is more accurate, can detect the slightest bit of pregnancy hormone in your blood, and you can test sooner than you can with a urine test.
Dr. Mark Trolice
- The article has been modified since this review was written.
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