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What does it mean when one pregnancy test is positive and one is negative?

Updated: Jan 19, 2022




When we initially decided to try for a baby, I had no idea what I was doing. I was tracking my ovulation on one of the free apps but, other than that, I didn’t have a clue what I was doing. I used home pregnancy tests when I started testing and I found that I got mixed results! Today, I will explain how it’s possible to get both positive and negative results on a home pregnancy test. How tests work The first thing we need to do is explain how pregnancy tests work. There are two main types of pregnancy tests: urine tests and blood tests. Both tests detect HCG levels in the body. HCG, or human chorionic gonadotropin, is the pregnancy hormone secreted which stimulates continued production of progesterone. Shortly after the egg is fertilized, the body begins to produce HCG in order to alert the body to continue to produce hormones necessary to sustain pregnancy. When the fertilized egg implants in the uterus (or fallopian tube, in the case of ectopic pregnancy), pregnancy hormone rises. HCG levels rise rapidly as the pregnancy progresses, doubling every 48-72 hours until they reach a plateau between 8 and 11 weeks of pregnancy. Most home pregnancy tests react to the level of HCG present in urine. The sensitivity of each test varies depending on the brand. However, most tests can detect the presence of HCG in urine about 12-14 days after conception, which is right around the time of a missed period. Blood tests have a higher sensitivity and can detect pregnancy earlier than most home pregnancy tests. They are usually performed by healthcare professionals in a doctor's office and are typically performed after a urine test (usually if those results are unclear). Since most women will be using home pregnancy tests, we will focus on three reasons why you might get varied (and confusing) results. Reason #1: Test or User Error I find that there are two main accuracy errors that occur while taking a home pregnancy test. The first is due to human error. Most tests give detailed instructions on how to take the test. Not following the instructions can lead to inaccurate test results. A very common example of this is not examining the test within the stated time frame. Most tests will instruct you to ignore any results that are read after 5 minutes. Once the urine test strip gets dry, you can often see the evaporation line on the test strip. This line is often faint and colorless but is usually the main culprit in false-positive test results! I strongly recommend thoroughly reading the test instructions prior to taking the test to avoid these types of mistakes. The second type of error is much simpler; you used a faulty test. There are so many home pregnancy tests on the market and the quality of the tests vary drastically. Even among the higher quality tests, there’s always a chance that the there is something wrong with the test itself. A faulty test may very well produce a false positive. If you’re not convinced that you are pregnant after the first test, I recommend “getting a second opinion” and taking a second test to see if the results are consistent with your first. Reason #2: Type of test Pregnancy tests come in all shapes and sizes! Most of them use some sort of dye to mark the Test and Control line on the chemical test strip. The most popular dye colors are blue and pink. While most tests do not mention dye color as significant, I found out the hard way that the type of dye you use matters! I started to feel symptoms in the very early stages of my pregnancy, so I started taking tests before my scheduled period (do not recommend doing this!). The first few tests I took were blue dye tests. On the first day, it was a clear negative test result. On the second day, there was a faint positive line, so I took another test later in the day to confirm. That one gave a false negative result. In my confusion, I went to message boards and google searches looking for answers. I discovered that pink dye tends to yield more accurate positive results. There’s some evidence that blue dye tests are more difficult to interpret and are more prone to showing that faint line and giving a false positive result. The next day, I took one blue dye and one pink dye test. The blue dye test showed a positive result again, but the pink dye test was negative! After waiting a few days, I took another pink dye test, which finally displayed a big fat positive! This brings us to another way the tests can defer. All tests have a “threshold” in which they will show a positive result based on HCG concentrations. Most tests will display a positive result with an HCG level of 25mIU/ML or greater. Some brands, like First Response, claim to have a higher sensitivity than other brands so it’s possible that their threshold is lower. In that case, if you took a test with a higher sensitivity first and then a lower sensitivity test, it’s possible to get a positive and then a negative. This happened to me with my second pregnancy! I took a First Response and got two lines, indicating a positive result! I wanted to announce to family so I took a ClearBlue digital test (because they show "pregnant" on the display). I suspect that the digital test had a lower threshold because it gave me a false negative. The best way to avoid confusion is to wait until your body has had enough time to build up HCG. I found viewing this chart from American Pregnancy helpful for understanding how HCG increases in the body. Reason #3: Early Miscarriage I hesitate to even include this one because I know how terrifying it can be to search the internet for answers about anything pregnancy related. Miscarriage is near and dear to my heart as I suffered my own loss at 6 weeks a few years ago. I will try to be as clear and concise as I can in order to not cause panic! Let’s say you followed all testing instructions, including waiting until your expected period to test, and got a positive (and accurate) result. If you followed up with another test (preferably the same type of test) days later and got a negative result, there is a chance you had a very early pregnancy loss, or a chemical pregnancy. Chemical pregnancies are more common than you think. Experts suspect that more women experience this than we know because it happens so early on that they just assume their period was late. A common reason women become aware of this loss is because they took one of the home tests very early on instead of waiting for a missed period. I want to include something that I think is very important when discussing this topic. If you’re like me, you were likely experiencing at least a few pregnancy symptoms by the time you got your positive pregnancy test result. I know from experience that when the miscarriage occurs and HCG levels drop, those symptoms slowly go away. A negative pregnancy test result is likely not the only indicator of pregnancy loss. A combination of a negative result, a decrease in pregnancy symptoms (low levels of HCG) and bleeding/spotting are a better indication of miscarriage. It's a good idea to discuss this with your healthcare provider. Conclusion Every person is different; it would be difficult to include all the ways in which you might get differing results while testing at home. Some medications and medical conditions have also been shown to impact pregnancy test results. It’s incredibly important to notify your healthcare provider when you get a positive result. If you get a positive very early on, your provider may do a blood test to confirm your results. I hope this was helpful! Leave a comment below and let me know what you think.

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