By Andrea Rodrigo BSc, MSc (embryologist) and Rebeca Reus BSc, MSc (embryologist).
Last Update: 01/31/2018

A pregnancy test attempts to determine whether a woman is pregnant or not by measuring the levels of the hCG hormone in urine. When a woman conceives, her body starts producing hCG, but the levels are still too low as to be detectable.

If you take a test on the first day of your missed period and get a positive test, it means that it’s probably about 15 days since you conceived. To be precise, doctors recommend that you take a test about 7 days after you miss your period to get an accurate result.

The different sections of this article have been assembled into the following table of contents.

Types of pregnancy test kits

Depending on the type of fluid used to measure the amount of hCG, we talk about home/urine pregnancy tests or blood pregnancy tests. Indeed, the pregnancy hormone can be detected in blood and urine.

Home pregnancy tests are often abbreviated as HPT, while blood tests are commonly called beta pregnancy tests.

HPTs are generally qualitative and can be done at home using a kit—that’s why they are also known as at-home pregnancy tests. On the other hand, blood tests should be done by a medical practitioner, and they can be qualitative or quantitative.

Learn more about the differences between both types of pregnancy tests in the following sections.

Home pregnancy tests

Home pregnancy test kits can be purchased at pharmacies or even at grocery stores. They are also referred to as predictors. HPTs can be found in the form of test strips, dipsticks, or digital tests. The main advantage of HPTs is that they can be taken at home—for most women, they are an easy-to-use, convenient and fast way of finding out if they are pregnant or not.

Home pregnancy tests attempt to measure the levels of hCG in urine in the woman. If she is pregnant, hCG will be present in her urine, thereby reacting to the antibodies of the test and showing a line or colored plus sign. Today, we can find digital tests such as the Clearblue pregnancy test with weeks indicator.

Qualitative home pregnancy tests have a sensitivity of between 25 and 50mIU/ml. In other words, they detect hCG hormone levels above this range.

The best time to take a urine pregnancy test is in the morning, with the first urine, because it’s the moment when the concentration of hCG is higher and is therefore more detectable. Drinking water or any type of fluids can affect the accuracy of at-home pregnancy tests, as they can dilute your urine and lower the levels of hCG.

Home pregnancy tests have an accuracy of 95-97% provided that you use them correctly.

Blood pregnancy tests

Blood or beta-hCG pregnancy tests work basically as home pregnancy tests: they measure the levels of hCG, but this time in blood instead of urine. In short, the following are the main differences between them:

  • The fluid examined
  • The sensitivity or detection threshold
  • The ability to determine how much hCG is present in the blood

Blood pregnancy tests can be quantitative and qualitative. While qualitative tests work exactly as urine pregnancy tests by showing a positive or negative result, quantitative tests are able to provide an exact value.

The result of a quantitative beta-hCG blood test shows an exact value that indicates the levels of hCG in blood. It should be examined by a medical practitioner who can determine whether this value means pregnancy or not.

There is some controversy regarding the detection threshold that should be taken as a reference to indicate whether the woman is pregnant or not. Broadly speaking, results under 10 mIU/ml are considered inaccurate.

If the result is below 10 mIU/ml, the most appropriate is to repeat the test. If the result doubles every 48 hours, then we can say that the pregnancy is progressing as expected. On the contrary, if the patterns of hCG decrease or stop increasing, it indicates that something is going on.

Errors with pregnancy tests

Both home pregnancy tests and blood tests are considered to be highly accurate in detecting pregnancy. However, they are not foolproof and unfortunately some factors can lead to pregnancy test errors. Learn to prevent them hereunder.

False negative result

During the first days of development, beta-hCG levels are still low. For this reason, there exists the possibility that the HPT is unable to detect them, as its sensitivity or detection threshold is higher than the current hCG level.

For this reason, we recommend that you wait approximately 15 days after having unprotected intercourse or undergoing an infertility treatment. After this time frame, known as two-week wait (2WW), the levels of hCG will be high enough as to be detectable and you will get an accurate result.

If you take the test too early, you are at risk of getting a false negative result. In short, it means that you are actually pregnant, but your hCG levels cannot be detected by the test yet. The result will turn into a positive if you repeat the test within a few days.

When a woman gets pregnant naturally, it is almost impossible to know the exact day in which conception took place. For this reason, an alternative option to find out if you’re pregnant or not is waiting for your period to show up. If you miss your period, you can take a test to confirm a possible pregnancy. Some gynecologists recommend that you wait at least 7 days after missing your period to take a pregnancy test.

Learn more about false negative results when testing for pregnancy in the following post: Pregnancy Symptoms but Negative Test, What Else Could It Be?

False positive result

On the other side of the coin, it is possible to get a positive result when you’re actually not pregnant. This situation is called a false positive and can occur due to a variety of reasons:

Ectopic pregnancy
Embryo implantation occurs, but in a location other than the womb. This type of pregnancy can lead to severe pregnancy complications. Embryo development should be arrested and the embryo removed as soon as possible.
Anembryonic pregnancy
Also known as blighted ovum. The embryonic sac develops, but there’s no embryo inside.
Chemical pregnancy
It is a miscarriage that occurs very early. Beta-hCG levels increase because there is implantation, but embryo growth stops within a few days.
Infertility treatments
When undergoing IVF or any other fertility treatment, it is likely that you have to use fertility drugs that contain hCG to trigger ovulation. If medication is still in your body, the remaining hCG can be detected by a pregnancy test and show a false positive.
Ovarian tumors
A highly elevated level of beta-hCG hormone when a woman is not pregnant can indicate the presence of cancerous cells.

Positive pregnancy test, what’s next?

The good news about pregnancy tests is that a positive result often means that you are indeed pregnant. It is a life-changing moment that comes with a wide range of surging emotions: joy, relief, panic… You have 40 weeks ahead to figure everything out, so pause for a second and relax.

First of all, after a getting a positive pregnancy test, the first thing you should do is call your doctor or midwife’s office in order to schedule your first prenatal appointment. Secondly, you can consider aspects such as your baby’s due date, decide who to tell and when, etc.

Now it is time for you to quit toxic habits that could be detrimental for your baby (including smoking, drinking alcohol, street drug consumption, etc.) and start a healthy lifestyle. A healthy pregnancy diet plan that includes the right amounts of protein, fruits, vegetables and whole grains is essential for your health and the baby’s.

FAQs from users

What is the price of a pregnancy test?

By Andrea Rodrigo BSc, MSc (embryologist).

At-home pregnancy tests typically cost between $8 and $20 and can be purchased at a drugstore or grocery store. Clearblue digital tests cost $18 on average.

Does toothpaste work as a pregnancy test?

By Andrea Rodrigo BSc, MSc (embryologist).

Actually, it doesn’t. According to a theory that is spreading across the Internet, this DIY test involves taking a cup of toothpaste and mixing it with a few drops of urine. If you’re pregnant, the toothpaste will supposedly begin to foam.

hCG contains amino acids and toothpaste is composed of a chemical called calcium carbonate. On the one hand, it is true that the combination between calcium carbonate and acid causes the substance to foam due to the production of carbon dioxide and water. So, this theory suggests that the presence of hCG can indeed cause toothpaste to foam.

However, urine contains acids as well, and this is the reason why this home pregnancy test doesn’t actually work. The acid in urine can cause the same reaction in the toothpaste whether you’re pregnant or not.

Recommended for you: Top 10 Historical Methods of Detecting Pregnancy.

When can I take a pregnancy test after a FET?

By Rebeca Reus BSc, MSc (embryologist).

Beta-hCG testing to find out whether pregnancy has occurred or not after a Frozen Embryo Transfer can be done within 15 days approximately, which allows you to get more accurate results. If done early, the test may give you a false negative or false positive result back.

There are cases, for example when blastocyst (day 5-6 of development) embryos are used, when the test can be done as early as 10 days after the FET. In any case, the general recommendation is to wait two weeks to take a pregnancy test.

Can you take a home pregnancy test while on your period?

By Andrea Rodrigo BSc, MSc (embryologist).

Yes, having your period during the test doesn’t interfere with the accuracy of the results. See also: Is It Actually Possible for You to Get Pregnant While on your Period?

What does a question mark on a digital home pregnancy test mean?

By Andrea Rodrigo BSc, MSc (embryologist).

It means that the test has malfunctioned for some reason. Don’t worry, there is nothing wrong with you. It can be due to a variety of reasons, including low battery, exposure to too hot or cold temperatures, humidity conditions, failure of the digital components, expired date, manufacturing defects… If this occurs to you, all you have to do is taking another test.

How quickly does a pregnancy test work?

By Andrea Rodrigo BSc, MSc (embryologist).

After doing the test, it may seem an eternity, but it usually takes 2-5 minutes to read the results.

Can you detect a pregnancy at 2 weeks?

By Andrea Rodrigo BSc, MSc (embryologist).

Yes, it may be. It depends on the sensitivity of the pregnancy test, though. If it is able to detect low levels of pregnancy hormones, you can get a positive result as early as 4-5 days before your period is due.

Will a pregnancy test detect miscarriage?

By Andrea Rodrigo BSc, MSc (embryologist).

Pregnancy tests cannot detect a miscarriage. However, because these days tests are able to detect very low levels, they may show a positive result in spite of having miscarried. hCG levels can still be high in the days or even immediate weeks after your miscarriage.

Learn more: hCG Levels After a Miscarriage or a Medical Abortion.

Can a pregnancy test detect ovarian cancer?

By Andrea Rodrigo BSc, MSc (embryologist).

Some types of cancer such as bladder and ovarian cancer can create small amounts of hCG, which can cause a false positive to occur on a pregnancy test. Only highly sensitive tests are able to do this.

What does BFP and BFN mean when talking about pregnancy test results?

By Andrea Rodrigo BSc, MSc (embryologist).

BFP is a slang abbreviation for Big Fat Positive, which refers to a positive pregnancy test result. On the contrary, BFN means Big Fat Negative, and translates into a negative result.

Suggested for you

Throughout this post, we have made various references to the pregnancy hormone, i.e. the hCG hormone. However, you may not know exactly what’s the function of this hormone or its normal levels. Here’s a complete guide to it, should you wish to delve deeper into it: What Are Normal hCG Hormone Levels during Pregnancy?

Also, even if you get a positive result on a pregnancy test, your pregnancy will be definitely confirmed on week 6 of fetal development as long as a gestational sac is seen through ultrasound imaging. Learn more about it here: What Is a Fetal Ultrasound?

Finally, we always recommend that you are patient and manage to “survive” the so-called two-week wait by being distracted with other things, especially if you’re on the 2WW after IVF embryo transfer. Get more info and useful tips to endure it here: What is the Two-Week Wait (2WW)? – Tips to Survive It.

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Authors and contributors

 Andrea Rodrigo
BSc, MSc
Embryologist
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia along with the Valencian Infertility Institute (IVI). Postgraduate course in Medical Genetics. More information
 Rebeca Reus
BSc, MSc
Embryologist
Degree in Human Biology (Biochemistry) from the Pompeu Fabra University (UPF). Official Master's Degree in Clinical Analysis Laboratory from the UPF and Master’s Degree about the Theoretical Basis and Laboratory Procedures in Assisted Reproduction from the University of Valencia (UV). More information
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2 comments

  1. Featured
    tulipan16

    Hello Sandra,

    I did the bood test but unfortunately my hCG level is 12. The nurse told me I have to repeat again the test because she could not confirm whether I’m pregnant or not.

    Could you please tell me your opinion? I read on the internet that a result higher than 5 is positive, but a very low hCG level is not a good sign.

    Thanks for you opinion.