What are the chances of a false positive pregnancy test

It can be incredibly stressful when you don’t know for sure if you are—or aren’t—pregnant. It’s common to want the answer as soon as possible, so your first thought might be to take a pregnancy test. While many tests boast a high accuracy percentage, it is still possible to get either a false positive or a false negative result.

Here we’ll discuss how pregnancy tests work and how inaccurate results can happen.

How a Home Pregnancy Test Works

When you get pregnant, the fertilized egg moves through the fallopian tube into your uterus, and it implants into the lining of the uterine wall; this is calledimplantation.

After implantation occurs, your body starts to produce a hormone called human chorionic gonadotropin (hCG). A blood test can detect the hCG pregnancy hormone about two to three days after implantation, which is just over two weeks from your previous period.

Early urine pregnancy tests are now highly sensitive and can detect hCG only five to six days after implantation, which is about three weeks from your last period and nearly one week before your next period is due. However, it’s recommended to wait until you’ve missed a period before you take a home pregnancy test to ensure you get the correct results you need. If you take a test before your period is due, it’s only 76% accurate, but if you wait to take a urine pregnancy test until your missed period, it’s 99% accurate.

Typically, your hCG levels will double every forty-eight to seventy-two hours from the time of implantation until you’re about ten weeks pregnant when the levels decrease and level out for the remainder of your pregnancy. This is whythe results are more likely to be accurate if you wait a few more days to take a test.

Most pregnancy tests work by holding the test stick in your urine stream or dipping it into a container of your urine. If the test detects a minimum concentration level of hCG, a positive result will occur. Any hCG concentration less than that level will show a negative result.

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How You Can Get a False Positive Pregnancy Test

A home pregnancy testcan show a positive result when you aren’t pregnant. This is known as a false positive, and they are rare, but they do happen. The following are the most common reasons why you may get a false positive: 

1. Some medications can cause a false positive pregnancy test.

Fertility drugs containing hCG can also lead to a false positive. Other medications that can cause a false positive result include:

• Some anti-anxiety medications
• Antipsychotics
• Some antiseizure drugs
• Parkinson’s disease medications
• Diuretics (“water pills”)
• Antihistamines
• Methadone
• Antinausea medications

2. A very early miscarriage occurred

After a pregnancy loss happens, there can still be detectable levels of hCG in your body at the time of a test. Approximately 50-75% of all miscarriages occur a couple of days after implantation, which is known as a “chemical pregnancy.” Since women get their periods as expected, many women don’t even realize they were pregnant unless they took an early pregnancy test.

3. You have a molar or ectopic pregnancy.

You will get a positive pregnancy test with a molar orectopic pregnancy because there is a fertilized egg that causes your body to produce hCG. However, in 99% of molar pregnancies, a fetus never develops—only abnormal tissue forms. An ectopic pregnancy implants outside of your uterus and is a medical emergency that requires immediate attention.

4. An evaporation line looks like a positive result.

Sometimes, a faint, colorless evaporation line can appear to be a positive result. The best way to avoid this is to read the results no later than the time the directions state.

5. The test is picking up hCG from a previous pregnancy.

If you were recently pregnant, you could get a false positive result from the test reading hCG still in your body from your previous pregnancy.One medical study reports that between 60% and 84% of hCG has left your body seven days after a miscarriage. This means that you could have 40% of pregnancy-level hcG a full week after miscarrying.

6. The test is expired.

The chemicals in an expired pregnancy test don’t work the way they’re supposed to, so the test can misread whether or not your urine contains hCG. Always use a test that isn’t expired.

How a False Negative Occurs

1. You took the test too soon.

The hCG concentration levels may not have been high enough for the test to detect. Waiting to take the test one week after you miss your period can help prevent this type of false negative.

2. There was not enough urine, or the urine was diluted.

This may have prevented the test from accurately detecting the hCG. To get the best results, use a home pregnancy test first thing in the morning to ensure that your urine is the most concentrated.

3. The test stick is broken or expired.

A false positive or a false negative can occur because the test malfunctioned, expired, or because of user error. That is why it is so important to follow the instructions exactly to get the most accurate results.

4. Your medications are causing a false negative.

 Certain antihistamines or diuretics can cause a false negative result.

5. There are pregnancy complications.

Ectopic pregnancies can also cause a false negative pregnancy test. Since an ectopic pregnancy is so serious, don’t hesitate to seek medical attention anytime you have pelvic pain.

Avail NYC provides free, self-administered pregnancy tests for those facing the possibility of an unexpected pregnancy. If you’re pregnant (or think you may be pregnant) and aren’t sure where to turn, meet with one of our advocates today.

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Avail NYC exists to be a safe haven for women and men facing an unexpected pregnancy or seeking support after an abortion. We are not a medical provider.

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