A false positive pregnancy test is very rare, but false negatives are extraordinarily common. That's comforting news to people struggling with infertility, especially those dealing with the chronic frustration of month after month of negative tests. Of course, averages and data about pregnancy tests don’t tell you much about your individual case, or what you can expect as you navigate the choppy waters of infertility. So what do you need to know about false positive and false pregnancy tests? Here’s an overview.
Pregnancy Tests: A Brief Tutorial
To understand why you might get a false positive or a false negative pregnancy test, you must first understand how pregnancy tests work. Pregnancy tests measure levels of human chorionic gonadotropin (HCG). This is a hormone that the body begins secreting when an embryo implants in the lining of the uterus. Implantation is therefore the medical beginning of pregnancy.
Women commonly have very low levels of HCG even when they’re not pregnant. So pregnancy tests detect HCG levels that are higher than usual, since higher HCG usually means a woman is pregnant.
HCG levels at the moment of implantation are very low, but they quickly double, and then triple, and then rise exponentially thereafter. So whether or not a pregnancy test can detect pregnancy depends on how much HCG a woman is producing. More sensitive tests can detect lower levels of HCG, and will therefore give a positive result earlier in a woman’s pregnancy.
A false negative pregnancy test is more common than false positive because a woman may get a negative test when HCG levels are too low. Very few things can mimic HCG, however, so as long as a pregnancy test is working correctly, the odds of a false positive test are virtually zero.
It's a myth that a woman is pregnant at the moment of fertilization, or that a test can be positive as soon as the egg is fertilized.
So testing a few days after having sex or ovulating will almost always produce a negative test result, even if the egg has been fertilized and the woman will soon test as pregnant.
False Negative Pregnancy Test: What Causes It?
False negative tests are common. In fact, many women get false negative tests before they get a positive. This is because the body’s HCG levels have not yet risen sufficiently high to indicate that a woman is pregnant. In many cases, a woman will later get a positive test if she just waits a few days.
The most common reasons for false negative pregnancy tests include:
- Not getting enough urine on the test.
- Not using urine that is sufficiently highly concentrated. Especially in the earliest days, it’s best to use the first morning’s urine. This urine tends to be more concentrated, and therefore has more easily detected HCG levels.
- Testing too early.
- HCG levels that are not rising rapidly enough. Sometimes a woman is pregnant, but her body does not have sufficiently high HCG levels to sustain the pregnancy.
- A test that is expired or otherwise defective.
- Not following the test instructions, such as by checking the test too early or late.
- Getting too much urine on the test stick, since this may affect the results and dilute a positive.
- Not correctly reading the test result. Even the faintest positive is still positive. If there is any line visible at all, read the test as positive.
- Testing at the wrong time. Some women chart their period only by counting days on a calendar. If your periods are irregular, you might have the dates wrong. Instead, it’s better to monitor your cycles, measure the day you likely ovulated on, and then test at least 10 days after this date.
- Using a pregnancy test that is not sufficiently sensitive. Some pregnancy tests can only detect HCG at very high levels -- levels that only occur several days into pregnancy. Read the test insert to determine how sensitive the test is. For the earliest possible positive result, pick the most sensitive test. This means selecting the test with the ability to detect very low levels of HCG.
False Positive Pregnancy Test: What Causes It?
To get a positive pregnancy test, many factors have to come into play. Like a perfectly orchestrated symphony, it’s easy for one component to be out of tune. That’s why false negatives are much more common than false positives. Even when a woman is pregnant, she might not get a positive result.
To get a positive test, a woman must:
- Use a sufficiently sensitive test.
- Test at the right time during her cycle.
- Test only after implantation.
- Have HCG levels that are rising.
- Correctly read the test, since even a faint positive is still a positive.
False positive pregnancy tests are very rare. Most women will never have one. In fact, false positives are so rare that if a woman does have a positive test and later has a negative test, she should assume she had a very early miscarriage. If you test positive, try testing again a day later. If that test is also positive, then the odds that you are not pregnant are nearly zero.
Some reasons a woman might have a false positive include:
- Allowing a test to sit for too long. This can cause an evaporation line that may look like a faint positive.
- Using an expired test. This can cause a positive line to appear even if the test is not positive.
- Having a chemical pregnancy. When this happens, the woman is pregnant, but has an early miscarriage.
- A recent miscarriage. HCG levels take a while to drop following a miscarriage. A woman who has recently had a miscarriage may still test positive on a pregnancy test.
- Taking certain fertility medications. Fertility medications that raise HCG or that are chemically similar to HCG can cause a false positive. Talk to a doctor if you take fertility medications, since a home pregnancy test might be less accurate.
If you have a positive pregnancy test, call your doctor since you are probably pregnant.
What if I Get Both a Positive and a Negative Pregnancy Test?
Some women get a negative test, and then later test positive for pregnancy. If this happens, believe the positive test. That’s because HCG levels take a while to rise, and the positive test may be because they’re finally high enough for a test to register.
If one test is positive and one test is negative, the best option is to wait a few days. Then take a third test. If that test is positive, you are probably pregnant. If the test is negative, or you continue to get a mix of positive and negative pregnancy tests, it could mean you are having a miscarriage or that your HCG levels are very low. Call your doctor. Your doctor can perform blood work to assess HCG levels and determine whether you are pregnant or not.
What if I get a Positive Test, and then Several Days Later it’s Negative?
Sometimes a woman gets a positive test followed by a negative. This sometimes happens when one test is less sensitive than another. But if a positive test is followed by several negative tests, this could mean the woman is having a chemical pregnancy.
You might read online that a chemical pregnancy is a pregnancy that never implants, or that it’s not a pregnancy at all. This is not true. Chemical pregnancies are shrouded in mystery and false information, especially online. A chemical pregnancy is in fact no different from any other pregnancy. The only distinction is that it ends with a miscarriage very early -- before an ultrasound can detect a heartbeat and before a doctor confirms pregnancy with an in-office test.
A chemical pregnancy is not your fault. Like any other miscarriage, it usually cannot be prevented. In most cases, it happens because there is a genetic anomaly with the embryo that would have made it impossible for the embryo to survive outside of the uterus. That doesn’t make the experience any less painful. You can grieve a chemical pregnancy just like any other miscarriage, and there is no “right” way to feel. The good news is that most women get pregnant soon after having a chemical pregnancy. A single chemical pregnancy does not mean there is anything wrong with your body, or that you are infertile.
Because a chemical pregnancy ends so early, many women do not develop other symptoms of pregnancy. But some do.
Some signs that you might be having a miscarriage of a chemical pregnancy include:
- Pregnancy symptoms that suddenly go away.
- A pregnancy test that was strongly positive, followed by progressively fainter positives on the same brand of test.
- Bleeding that looks like a period. For some women, the bleeding is slightly heavier.
Like any other miscarriage, your normal cycle should return shortly after a chemical pregnancy. However, sometimes the first period is slightly delayed following a chemical pregnancy.
Ectopic Pregnancies and Pregnancy Testing
Some online guides suggest that ectopic pregnancies may cause false negatives. This is partially true, but a bit misleading. An ectopic pregnancy is a pregnancy that grows outside of the uterus. It cannot grow into a healthy or normal baby, and can endanger the woman’s life. So this pregnancy must be removed as soon as possible.
Ectopic pregnancies do not develop or implant normally. So HCG levels may not be as high. Some women might get a handful of positive tests as well as one or two negative tests. If a woman experiences other symptoms of an ectopic pregnancy, such as pain in her side, vomiting, dizziness, or intense nausea, it’s important to check for an ectopic pregnancy. Because any pregnancy can be ectopic, it’s important to see a doctor if you have a positive test and think you might be pregnant.
How Soon Should You Test for Pregnancy?
The longer you wait to test after you ovulate, the more likely you are to get a positive result. Yet for most women, that two week wait is agonizing. So it’s best to balance the desire to test early with the knowledge that early tests are inherently less reliable.
To get the most accurate result, wait until at least eight days past ovulation (DPO). Implantation is extremely unlikely before this point, and even if it happens, HCG levels will not be sufficiently high to produce a positive test. For the most accurate results, wait until 11 days after ovulation, when slightly more than half of women get a positive result.
To increase the odds of an accurate result the first time:
- Use a highly sensitive early result pregnancy test. The most expensive tests are not necessarily the most reliable, so read the box to determine how sensitive the test is.
- Test with your first morning’s urine.
- Track your cycle so you know when you ovulated and can test on the right day.
- Precisely follow the test instructions, and do not read the test too early or too late.
Follow up with your doctor for confirmation if you get a positive result.
When to See a Fertility Specialist
If you’ve spent months or years dissecting pregnancy tests, googling for pregnancy symptoms, and getting nothing but negative tests, you already know all too well how painful fertility issues can be. The good news is that help is available, and the sooner you seek help, the greater your odds of a successful pregnancy will be.
Consider seeking help if:
- You have had two or more chemical pregnancies, or have had two or more negative tests preceded by positive tests.
- You have been trying for longer than a year to get pregnant, and the woman is under 35.
- You have been trying for longer than six months to get pregnant, and the woman is over 35.
- Either partner has a condition that can cause infertility, or has a previous history of infertility.
- The woman does not ovulate, or has very irregular menstrual cycles.
- The woman has had two or more miscarriages in a row.
Infertility can be agonizing. We understand. We’re here to help you find solutions as quickly, safely, and affordably as possible. Our experts set the standards in infertility care. Call today to learn more or to make an appointment with the Center of Reproductive Medicine!