August 23, 2014 - Life with Gremlins

Inaccurate Pregnancy Test Results: False Negative and Positive Causes

If you think you might be pregnant, chances are you already took a home pregnancy test. Whether you got a positive or negative test result, chances are you’re wondering whether that result was accurate. How common are false-negative pregnancy test results? False positives? What if there’s just a faint second line? Can you really trust what that tiny result screen is telling you? Can a pregnancy test be wrong? Let’s take a closer look at pregnancy test accuracy.

How do pregnancy tests work?

To understand the accuracy and possibility of false-positive pregnancy tests or false-negative pregnancy test results, it helps to understand how a pregnancy test kit works.

Home pregnancy tests detect a hormone called Human Chorionic Gonadotropin (hCG), also known as the pregnancy hormone. hCG ensures progesterone production continues to maintain your uterine lining during pregnancy. Cells that will later form your placenta immediately begin releasing this hormone after the fertilized egg implants in your uterine lining.

While a blood test can detect any level of hCG, most home urine tests have a threshold of about 20-25 mIU/mL. However, the most sensitive home test (First Response Early Result) can detect levels as low as 5.5 mIU/mL. Either way, these levels aren’t typically reached until 3 to 5 days after implantation. You can read more about when to take a pregnancy test for the most accurate results here.

How common is a false-negative pregnancy test result, and why do they happen?

By definition, a false-negative pregnancy test is a test that says not pregnant when you are pregnant. False-negative pregnancy test results are actually rather common in early pregnancy as they have numerous causes.

For instance, a retrospective cohort study performed by researchers at the Boston University School of Medicine in 2021 found that among 10,924 urine hCG tests performed during emergency room visits, 1.6% yielded a false negative result. Rates were higher among women suffering from ectopic pregnancy (3.6%), a condition where a fertilized egg implants outside the uterus, such as in the fallopian tube or ovary, as hCG levels are typically lower than normal in these cases.

Keep in mind these rates were in a hospital setting with the instructions for completion and test results being read by a trained medical professional. There currently are no studies on the prevalence of false-negative results in the real world, but as you can imagine, they are likely much higher.

You may receive a false-negative pregnancy test result if:

-You tested too early, and hCG levels were not yet sufficient.
-You didn’t test too early, but hCG levels were still insufficient.
-Your hCG levels were too high (the hook effect).
-Your urine was too diluted.
-The test was done incorrectly. For example, perhaps the test was not saturated entirely in urine.
-The test was expired.
-The test results were read too long after testing.
-You misread the result (1 in 4 women are said to misread traditional pregnancy tests).

If you think you may be pregnant or are experiencing symptoms of pregnancy and get a negative pregnancy test result, it’s best to test again in around a week. If the second test is negative, but symptoms persist (including missed periods), it’s time to consider seeing your healthcare provider. In the meantime, you might also find our page on missed periods with a negative test helpful.

How common are false-positive pregnancy test results, and why do they happen?

True false-positive pregnancy test results, meaning the test result is read properly, are far less common than false-negative results, but they can happen. As an example, in the substantial equivalence laboratory report for First Response’s Gold Digital Pregnancy Test, which included 575 tests, only .18% (a single test) yielded a false-positive result. These pregnancy tests feature an easy-to-read digital result screen that shows a plus sign and yes for positive results or minus sign and no for negative results. In the review, 98% of test-takers read their test result correctly, meaning the .18% is a fair representation of the prevalence of real false-positive test results.

You may receive a false-positive pregnancy test result if:

-You misread the result. Some tests will leave a very faint line as the urine passes the test indicator window that can appear to make the test positive. Tests that come back with very faint results should be re-done in a few days to a week.

-You got an evaporation line. Some tests, usually the cheaper variety, will show a “ghost line,” which is typically colorless or thin even if you are not pregnant. Reportedly among conception circles, this is more common in blue-dye tests. If you have to squint to see the line, test again in 48 hours.

-You waited too long to read the results. In most cases, an aged pregnancy test result will appear negative, but it is possible for a negative test to look positive after the recommended read time. This line is more likely to be colorless.

-You were pregnant but aren’t anymore. Even if a pregnancy fails within days of implantation, hCG is still produced. This is called a chemical pregnancy. Chemical pregnancies often present with no other symptoms, unlike a miscarriage, which typically comes with heavier bleeding and cramping at the least. Early pregnancy loss is common, occurring in roughly 1 in 4 pregnancies.

-You are taking a drug that contains hCG or causes analytical interference in hCG testing. A few select fertility medications, such as Pregnyl and Profasi, contain hCG, but a longer list of common medications can interfere with the accuracy of hCG testing, including anticonvulsants, like Carbamazepine, and antipsychotics, like Chlorpromazine and Thioridazine. Your pharmacist should be able to offer guidance as to whether any medications you’re taking could lead to inaccurate pregnancy test results. Note that birth control use can not cause a false positive.

-You have cancer or an ovarian cyst. Certain medical conditions, including gestational trophoblastic diseases (molar pregnancy), breast, ovarian, and lung cancer, and ovarian cysts, can release hCG, which could result in a false-positive test result. If you receive a positive pregnancy test, don’t wait to visit your health care provider, as these conditions, among others, such as ectopic pregnancy, can become life-threatening if left untreated.

-You are post-menopausal or just got a really sensitive test. Even women who aren’t pregnant have low levels of hCG. These levels are higher in some post-menopausal women, and the accuracy of pregnancy tests varies. In very rare cases, a test will read positive as a result of normal hCG levels in the body.

Can you trust home pregnancy tests?

A home pregnancy test is a good first step in determining if you are or aren’t pregnant if you miss a period or are experiencing other pregnancy symptoms. Keep in mind, though, they aren’t without fail, making them a better first step than last. When in doubt, see a health care provider, especially if what caused that doubt continues.

You may also find helpful:
Can you have a period and be pregnant?
Early Signs of Pregnancy Quiz
DIY Pregnancy Tests: Are They Accurate?

Can You Drink Expired Milk?

We’ve all done it. Crusty eyed and in desperate need of coffee, you reach into the refrigerator, grab the milk and pour the last creamy drops into that steaming, delicious, alarm clock of caffeine. You draw the cup to your lips and hazily notice the expiration date on your milk pressing the snooze button. Its two days past the sell by date, is it ok? If it’s expired can you drink expired milk? If you can’t drink expired milk, is there any use for it other than dumping your hard earned money down the drain?

When is milk expired?

Most milk containers contain at least one date, called a sell by date, stamped usually near the top. Some milk takes the guess work out by also containing an expiration date as well though. If your milk only has a sell by date, this is the date the store has to sell the milk by or remove it from the shelves. Generally, if kept cold, milk will still be good three days to a week after this date. The colder milk is kept the longer the milk will stay good.

Milk that has expired will sour. Usually good sniff is sufficient to tell if milk is expired as a result. If you can’t tell by the smell, take a small sip, it will taste just odd, bitter, or well, sour. Another dead give away is curdling or lumps. The bitter taste will precede chunks, at this point its sometimes called “Bajaxed.” Sometimes the surface of bajaxed milk looks almost textured, like it’s begun to separate a bit.

Can you drink expired milk? What happens if you do?

If the milk has just begun to sour, you may not notice the bitter taste and continue to drink, chances are this will have no effect on an adult. On the other hand, a child could possibly continue drinking milk that has gone chunky even. If so symptoms could be equal to food poisoning, but a tummy ache is more likely if any side effects present at all beyond a gross flavor. Food poisoning lasts around 48 hours and can include; vomiting, nausea, stomach pain, and diarrhea. In severe cases, blurred vision and dizziness. If any symptoms exceed 48 hours medical care should be found.

Ok, so the milk is expired, is there anything I can use it for?

Soured milk is great for baking. In fact, some recipes actually call for it. It’s also an ingredient for many ethnic foods. You could make your own yogurt. (recipe) Bajaxed milk makes wonderful cocoa when boiled too. Some people will pour soured milk into septic tanks to create decomposing bacteria. Soured milk can be used to make paint and was by American pioneers. (I have a link for that too!) Expired milk is also better for milk baths, because the increase in lactic acid, though you may want to catch it before it curdles for that use. It can be used to clean leather and will restore its luster. Expired milk is even rumored to be wonderful for trees and rose bushes, but I couldn’t find back up information of that one.

So there you have it, and answer to can you drink expired milk–you likely don’t want to drink straight expired milk, but as an adult it probably won’t do much to you unless its really soured. If it is expired, no worries it has a world of uses.

Can Pregnancy Symptoms Predict Baby Gender?

Like many multi-child mothers with same-sex children, I have my eye on a certain gender this pregnancy. While I’m not much for wives tales, one particular suggestion caught my more scientifically-swayed side. It’s often said that pregnancy symptoms can predict baby gender. This is suggested to be due to the difference in hormones a girl or boy baby require. A quick poll of pregnancy symptoms and sexes of said pregnancy’s offspring on my Facebook page for parents, which boosts about ten thousand, however, found no theme in pregnancy symptoms and sex of the child. Still curious, I voted to dig just a little deeper.

Do girls and boys really cause different hormone surges during pregnancy? Is it possible to predict baby gender based on pregnancy symptoms?

The real test of this wives tale lies solely in the supposed scientific basis. If it’s false that hormone levels don’t vary with the sex of your baby, then the whole idea is blown to diaper bits. Luckily, it isn’t false, at least based on studies done so far. Research does currently support that hormone levels vary depending on the sex of your baby as early as three weeks along.

Hormones high in girls to predict baby gender:

HCG–yes, that’s the same hormone that made your pregnancy test positive-has been shown to often be higher in mothers who are pregnant with little girls. This is why studies have also proven that women pregnant with girls experience higher rates of the pregnancy symptoms nausea and morning sickness. HCG is a known culprit of both.

Hormones high in boys to predict baby gender:

Testosterone: Beginning around week 7 and peaking around week 9 to 11, a surge of testosterone is actually what prompts the development of male sex organs. The lack thereof causes the development of female organs by the way, not a surge of estrogen or “female hormones.” While no studies were found to confirm a correlation between higher testosterone and indicative pregnancy symptoms, those would include increased hair growth, darkening of body hair, and/or excessive acne or oily skin.

There are, of course, other hormones secreted during pregnancy, and even by your baby in later weeks. These two are just the two that are suggested to surge, so to speak, sufficiently to cause a differentiation in symptoms. Keep in mind of course, that many women who experience horrible morning sickness have little boys, and many women who have hair changes or acne have girls.

Gender determination based on hormone levels hasn’t been proven reliable even with blood samples (excluding a DNA test). So, none of the above is really better than a wives tale, but it may at least offer you something to ponder while you wait for that gender-determining ultrasound moment.

Did this wives tale accurately predict baby gender for your pregnancy? It was for all three of my boys and for the little girl we are now expecting. You may also enjoy our baby gender determination quiz based on the science of gender swaying.