Can antibiotics cause a false positive pregnancy test

In this article, Dr. Gleaton shares how pregnancy tests work, what medications could affect pregnancy test results, and when to talk to your doctor.

Taking a pregnancy test can be a nerve-wracking experience, and you want to be confident when reading the results. We’ve already covered the most common questions in our guide to peeing on a stick (POAS), but now it’s time to cover what medications could affect your pregnancy test results. 

How medications affect pregnancy test results

To recap, pregnancy tests work by detecting the pregnancy hormone, hCG, in urine. If the pregnancy test detects a certain level of hCG, it will show a positive result. hCG is made when a fertilized egg implants in the uterus, usually around 10 days after conception, and increases every day in early pregnancy. 

If you’re taking a medication that contains or affects hCG production, it’s possible that you could receive a false pregnancy test result. This means you could receive a positive test without actually being pregnant, or (more rarely) a negative test when you are pregnant. 

What medications may affect pregnancy test results

Most medicines, including over-the-counter and prescription, should not affect the results of a home pregnancy test. But medicines that have the pregnancy hormone hCG in them may give a false positive test result. This includes some medicines given for infertility treatments—often called the “trigger shot” and sold under the brand names Novarel, Ovidrel, Pregnyl, and Profasi.

What to do if you suspect medication is affecting pregnancy test results

If you suspect your medication may be affecting your pregnancy test results, you should consult with your doctor. Your doctor can run a blood test and perform a transvaginal ultrasound to give you a definite answer on whether or not you’re pregnant. 

If you’re on medications for fertility treatments, it’s important you don’t interfere with your medication routine without supervision from your doctor.

When to call your doctor

If you believe you’ve been getting multiple false positives or false negatives, you should see your doctor. This is especially important if you are participating in fertility treatments or are showing symptoms of pregnancy, but receiving negative test results. 

Some false pregnancy test results could be a result of a hormone imbalance, including PCOS, a chemical or ectopic pregnancy, evaporation lines, and more. Make sure you’re following your test instructions correctly to avoid any user error. We made sure our early-result pregnancy tests are simple and easy to use with jargon-free instructions to get the clear answer you need.
 

Follow instructions and know when to seek advice

It’s important to follow directions exactly when taking pregnancy tests, otherwise you could end up with a false positive or false negative result. If you’re following the instructions correctly and you’re still concerned that your results are incorrect, you should consult your doctor. Many fertility medications and occasionally some OTC or prescription drugs could cause a false pregnancy test result, but a blood test and transvaginal ultrasound will help rule this out. 

Home pregnancy tests work by detecting the presence of human chorionic gonadotropin (hCG) in a urine sample. There are a few things that can cause a false negative reading, namely the improper use of the test, testing too early, using an expired test, or diluting the urine by drinking too much water in advance.

There are also a few medications that can interfere with the result. Rather than triggering a false negative, they can sometimes return a false positive reading, leading to you believe that you are pregnant when, in fact, you are not.

HCG Medications

Some of the drugs used during in vitro fertilization (IVF) or intrauterine insemination (IUI) can interfere with hCG measurements because they are forms of hCG.

Women unable to ovulate on their own are often prescribed a fertility medication such as Clomid (clomiphene citrate). After completing the five-day course, a "trigger shot" of Pregnyl, Ovidrel, or Novarel may be used to mature eggs and induce ovulation.

Typically, it takes about 10 days for the synthetic hCG in these trigger shots to clear from your blood and urine. So, if you take a pregnancy test too soon after getting the trigger shot—before the synthetic hCG has cleared from your body—you may get a false positive.

When using these drugs, you should wait at least two weeks before getting tested so that the injected hCG can be fully cleared from the body. Most doctors, however, advise against home testing and ask that you come to the office to get a blood-based test. While the blood test also detects hCG, it is far more accurate than a home urine-based test.

HMG Medications

Another type of injectable drug, known as human menopausal gonadotropin (hMG), can be used to aid in ovulation. HMG is made up of two hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Common hMG medications include Menopur, Pergonal, and Repronex.

When used in the IVF or IUI setting, the LH component can also trigger a false positive result but for different reasons. While an hCG injection triggers a false positive by introducing synthetic hCG into the system, a surge in LH can do the same because the two hormones are so structurally similar.

False Negative Pregnancy Tests

If you've taken a pregnancy test and believe that a negative result is wrong, the best thing to do is wait for at least two days and test again. Often, if you test too early, the hCG levels will be below the threshold of detection of many home pregnancy tests.

HCG levels are typically expected to double every two days if you are pregnant. If you are pregnant and you wait another two days before testing again, the levels should double again, increasing your chance of an accurate reading.

If, on the other hand, you are undergoing IVF or IUI, it is best to follow your doctor's instructions and wait to get an in-office blood hCG test. While having to hold off two weeks can add anxiety to an already emotional process, the risk of a false positive test may be more upsetting than waiting for an accurate reading.

By Robin Elise Weiss, PhD, MPH
Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor.

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