When to Take a Pregnancy Test to Avoid False Negative or False Positive Results

When to Take a Pregnancy Test to Avoid False Negative or Positive Results

Knowing the right time to take a pregnancy test can help ensure that you get the most accurate results. Taking a home pregnancy test too early can result in a false negative result. This means that you have conceived but there is not enough hCG hormone (the pregnancy hormone) in urine to show a positive result. In some rarer instances, the home testing kit can give a positive reading even though you are not pregnant (false positive). In other cases, the urine pregnancy test may show a faint positive line or evaporation line which usually indicates that you are pregnant. However, in these cases it’s best to repeat the test to make sure it’s not a false positive result.

Many women take a pregnancy test when they notice the first signs of implantation. If you have a regular menstrual cycle, then usually the best time to take a urine home pregnancy test would be the first day after your missed period.

In this article, I will look at when the optimal time to take a pregnancy test is. I will also look at the meaning of evaporation line (evap line) – a faint positive line that some say give a false-positive test result. Getting accurate results from your home pregnancy test can help to relieve much of the anxiety while waiting to find out if you are pregnant or not.

How Urine Home Pregnancy Tests Work

Home pregnancy tests work by checking the amount of the hormone human chorionic gonadotropin (hCG) in urine.

Scientists in Germany report that a few days after you become pregnant, your body secretes the hCG hormone. This hormone steadily rises and can usually be detected about 8 days after conception. However, knowing when the earliest time to get accurate results from a home pregnancy testing kit can be a challenge. The scientists say that hCG concentrations can vary depending on a women’s menstrual cycle.1

The journal Clinical Chemistry says that manufacturers of pregnancy test kits claim that the results are 99% accurate when taken on the first day after a missed period. Some home pregnancy tests claim to be sensitive enough to give accurate readings even earlier.2

When to Take a Pregnancy Test

For the most accurate results, the best time to take a pregnancy test to avoid a false negative would be at least a day after your missed period. However, some pregnancy tests are not sensitive enough and they may show false negative results if taken too soon after implantation. If the test is negative, take the test again to see if a positive result shows up. If you still get a negative result 1 week after the missed period, this virtually guarantees that you are not pregnant.

Let’s look in more detail at the best time to take a pregnancy test after implantation to get accurate test results.

According to the journal Human Reproduction, some pregnancy test kits can detect positive levels of hCG as soon as 3 or 4 days after implantation. Most testing kits (about 98%) should be able to give accurate results about 7 days after fertilization has occurred. Usually, the results from a pregnancy test a week after your missed period will be 100% accurate (if you have a regular menstrual cycle).3

However, it’s important to remember that the best time to get accurate results from a home pregnancy testing kit varies from woman to woman. OB/GYN Dr. Traci C. Johnson says that some of the factors that can affect the accuracy of a pregnancy test can include:4

Ovulation. Depending on your menstrual cycle, ovulation can occur between day 11 and day 21 of your cycle.

Time of implantation. If sperm reach an egg to fertilize it, this can take place between 6 and 12 days after ovulation. When implantation occurs, you may notice some of the signs of implantation.

hCG levels in urine. A study published in the Henry Ford Hospital Medical Journal reports that hCG hormone levels fluctuate in individual women.5

Sensitivity of the pregnancy test. Some pregnancy tests are only 99% accurate when the urine sample is taken the day after a missed period. Other tests say that they can only detect pregnancy about a week or so after implantation. This means that if you take the pregnancy test too soon after fertilization you may get a false negative result.

If you are unsure about your ovulation cycle, take the pregnancy test as soon as 1 day after your missed period. A negative test result 7 days after your missed period virtually guarantees that you are not pregnant.

False Negative Pregnancy Test

Sometimes, a negative result on a pregnancy test can be wrong. A “false-negative” result on a pregnancy test means that the test result is negative when you are actually pregnant. This usually happens if you take the home pregnancy test too early before levels of hCG have become detectable.

What causes a false negative pregnancy test?

Here are some of the reasons for a false negative pregnancy test:

Testing too early. Taking the pregnancy test too early usually results in a false negative pregnancy test. Levels of hCG need to reach detectable levels before the second line on the pregnancy test results shows positive.

Home pregnancy test is not accurate enough (despite its claims). Although most home kits for pregnancy testing claim 99% accuracy, many are not that accurate. For example, a report from 2014 stated that the reported accuracy of pregnancy test kits is only under ideal laboratory conditions. In reality, the accuracy of home testing kits is much lower.1

In fact, a study in the Journal of the American Pharmacists Association found that many standard urine pregnancy test kits showed false pregnancy test results when testing for pregnancy on the first day after a missed period.7

Molar pregnancy. Molar pregnancies are quite rare and happen when a genetic error causes abnormal tissue to grow in the uterus. The journal Gynecologic Oncology Reports found that molar pregnancies can cause abnormally high hCG levels.

In some cases the hCG levels are much higher than what the testing kit is designed to measure and it may completely distort the test results. This interference is called the “hook effect”. As a result of the “hook effect”, the pregnancy testing strip can show a false negative result despite having extremely high hCG levels in urine.

Diluted urine. Most urine pregnancy test kits recommend using a urine sample of your first pee in the morning. This is because hCG will be most concentrated. The British Journal of Obstetrics and Gynecology found that some pregnancy kits were ineffective at detecting hCG in diluted urine.9

Incorrect interpretation of the test result. In some cases the home pregnancy test may show a faint positive line. Some women consider it as an evaporation line (evap line) and interpret it as a negative result. However a positive result (even a faint line) will only show up when hCG is present in the urine. You can find more information about faint positive line or evap line in the second part of this article.

How to avoid a false negative pregnancy test

Doctors from the Mayo Clinic give the following tips to avoid a false-negative pregnancy test result:6

  • For the most accurate results, take the pregnancy test one week after your missed period. At this time the level of hCG in your urine is most likely to be detectable.
  • Give enough time for the test to work and don’t check the results too early or too late.
  • Take the pregnancy test first thing in the morning when your urine is the most concentrated.
  • You should also carefully follow the instructions on the home testing kit.

False Positive Pregnancy Test

It is possible to have a “false-positive” pregnancy test, although these are rarer than false-negative ones. Doctors from the Mayo Clinic explain that a false-positive result on a pregnancy test is when the result is positive but you are not pregnant.6

What causes a false positive pregnancy test?

Chemical pregnancy. A successful fertilization followed by a very early termination is also called a chemical pregnancy. The Journal of Clinical Medicine Research explains that a false-positive test result can mean that the woman was only pregnant for a few days. Very often, the spontaneous termination occurs without any symptoms and the menstrual period happens as usual.10

The false positive result occurs because there are still high-enough levels of hCG in the urine that trigger a positive, but false result. In some cases the home pregnancy test may show a faint positive line due the very low hCG levels caused by the chemical pregnancy.

Too many white blood cells. Another reason why the test kit shows a positive result when you are not pregnant is if you have too many white blood cells (Leukocytes). According to research published in the Annals of Laboratory Medicine, leukocytes in urine can result in a false-positive result when using a home pregnancy testing kit.11 Leukocytes in urine are often present if there is a urinary tract infection.

Medication side effect. Certain medications like tranquilizers, hypnotics, or anticonvulsants can cause a false-positive pregnancy test.4

Ectopic pregnancy. Your hCG levels will rise and show a positive result if you have an ectopic pregnancy.12 Although not a false pregnancy, an ectopic pregnancy occurs when the fertilized egg grows outside the uterus. The growing fetus needs to be removed because ectopic pregnancy can put the woman’s life at risk.

How to avoid a false positive pregnancy test

As with all home pregnancy kits, it’s important to carefully follow the instructions to prevent a false positive pregnancy test. However, in many cases, there are some underlying causes of a false-positive result that are not connected with testing methods.

Faint Line on Pregnancy Test – What Does it Mean?

What can it mean if you notice a very faint line on your pregnancy test? Some people refer to this as an evaporation line (or, evap line) which, they say, indicates a false-positive test result. However, there is much discussion on what exactly the faint positive line on a pregnancy test means.

Some people say that the faint colored second line is a chemical reaction to dried urine on the testing stick, hence it’s called evaporation line. However, clinicians say that this is not the case because hCG testing kits are designed to only react to the pregnancy hormone. They don’t change color because of ink or a reaction to certain chemicals in urine.

The best way to confirm a pregnancy if you notice a very faint line on a pregnancy test is to repeat the test a few days later. You should also try to be aware of any implantation signs or early pregnancy.

Of course, even the faintest of line on a pregnancy test can be a false-positive for any of the reasons already mentioned in this article.

Faint positive: does it mean you are pregnant?

If you take a home pregnancy test and there is a faint positive line, it’s highly likely that you’re pregnant. In many cases you’ll notice a clear and evident positive line after taking a home pregnancy test. But in other cases, the positive line may appear very faint or faded. In most cases, faint positive line is caused by low levels of the pregnancy hormone human chorionic gonadotropin (hCG).

A second line, even a faded line on a pregnancy test, shows that levels of the pregnancy hormone hCG have been detected. In the majority of cases, this means that you are pregnant.

Evaporation line on pregnancy test

Some people say that a so-called “evaporation line” on a pregnancy test can be mistaken for a false-positive result. They claim that this false positive line can appear after a few hours when urine evaporates from the stick. The evaporation line will look very faint and will be somewhat fainter than the control line. This may cause you to mistakenly think you’re pregnant.

However, as I’ve already pointed out, a second positive line, even a very faint positive line, indicates levels of hCG in urine. To be sure that the “evaporation line” means that you are pregnant, you should take a second test a few days later. You should also take the pregnancy test first thing in the morning and check the results within the appropriate time frame.

Early Signs of Pregnancy Soon After Implantation

There are usually signs of early pregnancy that you can spot even before you miss your next period. Very often, noticing these signs means that it is time to start thinking about taking a pregnancy test. What are the signs of early pregnancy that you can spot before your period is due?

Implantation bleeding. Doctors from the American Pregnancy Association say that implantation bleeding looks like very light brown or pink spotting. The light bleeding happens as the fertilized egg implants itself into the lining of the uterus. This can be one of the earliest signs of pregnancy.13

Implantation cramping. You may feel some mild or light cramping in the lower abdomen before your period when implantation occurs. According to an expert in gynecology, Dr. Trina Pagano, cramping can happen at the same time as implantation bleeding. Because the implantation cramping happens just before the period, you could easily mistake them for menstrual cramps.14

Increased vaginal discharge. Another of the earliest signs of pregnancy soon after implantation is a change in vaginal discharge. The American Pregnancy Association says that vaginal discharge during pregnancy is milky white with a thin consistency.15

Increase in basal body temperature. If you regularly record your basal body temperature, you will notice that it will not drop after ovulation if you are pregnant. Doctors from the Mayo Clinic say that an early indicator of pregnancy is a high basal body temperature for at least 18 days after ovulation.16

Frequent urination. Researchers from the National Institutes of Health report that the pregnancy hormone hCG stimulate blood flow to the pelvis in the first weeks of pregnancy. This can result in a frequent need to pee.17

Other signs of early pregnancy:

When to See a Doctor

If your hCG urine pregnancy test shows a positive result, you should make an appointment with your doctor as early as possible. Your doctor will arrange for a blood test to check levels of hCG to confirm pregnancy. Receiving prompt pre-natal care is the best way to care for your health and the health of your baby.

Related articles:

Medical Sources

  1. Geburtshilfe Frauenheilkd. 2014 Jul; 74(7): 661–669.
  2. Clin Chem.2001 Dec;47(12):2131-6.
  3. Hum Reprod.1992 May;7(5):701-10.
  4. WebMD. Pregnancy tests.
  5. Henry Ford Hosp Med J.1982;30(4):207-13.
  6. MayoClinic. Getting pregnant.
  7. J Am Pharm Assoc (2003).2005 Sep-Oct;45(5):608-15.
  8. Gynecol Oncol Rep. 2017 Aug; 21: 34–36.
  9. Br J Obstet Gynaecol.1998 Apr;105(4):462-5.
  10. J Clin Med Res. 2013 Aug; 5(4): 269–274.
  11. Ann Lab Med. 2012 Mar; 32(2): 167–168.
  12. J Fam Plann Reprod Health Care. 2011 Oct; 37(4): 231–240.
  13. AmericanPregnancy. What is implantation bleeding?
  14. WebMD. Pregnancy symptoms.
  15. AmericanPregnancy. Vaginal discharge during pregnancy.
  16. MayoClinic. Basal body temperature for natural family planning.
  17. NICHD. What are some common pregnancy signs?

Healthy and Natural World