The Truth About Pregnancy Tests

Amber Hewitt, MSPAS, PA-C 
Amber Hewitt, MSPAS, PA-C 
Published: March 14, 2022Updated: March 28, 2022
Reviewed by Dr. Lisa Czanko MD, MPH

Counting and recounting the days on your period-tracker, going over details of the last time you had sex, feeling a whole bunch of butterflies: your period is late. And it’s all you can think about. You scribble pregnancy test? in your agenda. In red ink.

Maybe you slip into a convenience store late at night so you can finally get some sleep. Maybe you enlist a brave friend to purchase a test for you Mission: Impossible style. Maybe you stride into that store with zeal, excited about the possibility of becoming a parent. But when there’s a whole aisle dedicated to different kinds of pregnancy tests, it can be overwhelming.

Regardless of the reason for doing so, taking a pregnancy test can bring up a LOT of emotions. Will you see one line? Two? A smiley? A frown? Will someone pop out of a cake? We’re here to help get you some clarity about how exactly pregnancy tests work.

In this article, we break down the science behind peeing on a stick.

Understanding Conception

It’s easy to forget that periods are more than monthly inconveniences. To understand how pregnancy tests work, let’s review how the menstrual cycle works to prepare the body for pregnancy.

Menstrual Cycle Overview

A menstrual cycle refers to the first day of one period to the first day of the next period. The average range of one cycle is 21-35 days.

Through a series of coordinated hormonal changes, the first part of the cycle allows an egg to mature in the ovary. Next, ovulation occurs, which is when the egg is released from the ovary. Ovulation typically occurs about 14 days before someone’s period is due. 

Late Periods

Conception occurs if sperm is present and fertilizes the released egg. After conception, various hormones get the uterus lining ready to support a pregnancy. Period bleeding will not happen, as all of that blood and nutrients are being used. People will notice this as a “late” period and may reach for a pregnancy test.

About eight to ten days after fertilization, the fertilized egg nestles into the lining of the uterus. This is called implantation. Once this occurs the cells that will become the placenta start to make a hormone called human chorionic gonadotropin (hCG). The level of hCG will continue to rise through the first trimester of pregnancy. This hormone is what is detected on a pregnancy test. 

If you’ve ruled out pregnancy, late or missed periods can also point to other medical conditions, such as stress, weight changes, PCOS, anorexia, and thyroid diseases. It’s essential to pay attention to unusual symptoms and see your medical provider for evaluation.

Although some urine pregnancy tests are marketed as being able to show a positive result before a missed period, taking a test the day a period is missed helps ensure better accuracy.

Understanding Pregnancy Tests

A late period can cause a lot of stress or excitement. And getting clarity around figuring out if you are pregnant or not can bring some comfort.

There are two basic pregnancy tests: a urine pregnancy test and a serum (blood) pregnancy test. The over the counter pregnancy tests are urine tests. Blood tests can be ordered by a medical provider.

How Urine Pregnancy Tests Work

A urine pregnancy test has a special kind of paper that absorbs urine and reacts if there is hCG present in the urine. A person may pee directly on the stick or dip the stick in a cup of urine. After a few minutes, a result is displayed.

The test often has a window that contains a control line. This line shows up whether the person is pregnant or not and is there to let them know that the test is working.

The second part of the window typically changes color if hCG is present. Depending on the particular test that is used, this can appear as a second line that is the same color as the control, a line that is a different color, a plus sign, or a happy face. There are also some tests that show the words “pregnant” or “not pregnant.”

Although some urine pregnancy tests are marketed as being able to show a positive result before a missed period, taking a test the day a period is missed helps ensure better accuracy. If a period still doesn’t come, a person can consider taking a second test a few days later, even if the initial test was negative.

How Serum (Blood) Pregnancy Tests Work

A serum hCG test is usually done at a lab or medical provider’s office. This can often be done at the same time as other blood tests if needed.

After blood is taken from a vein, it’s then run through a machine that can tell whether hCG is present.Some blood tests, called quantitative hCG tests, tell how much hCG is present in the blood. 

Sometimes a medical provider will recommend doing a quantitative hCG blood test if there is a concern for the viability of an early pregnancy or if fertility treatment was used to help someone conceive.

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When To Reach Out For A Pregnancy Test

Because hCG is only produced after implantation occurs, there is no pregnancy test that is able to immediately detect whether conception has occurred.

While many urine tests can detect hCG around eight days after fertilization has occurred, it is generally recommended to take a pregnancy test on the day the period is normally expected to start to ensure the greatest chance of accuracy. If there is a faint positive line on a urine pregnancy test, it’s still considered a positive. If you have any questions be sure to contact your medical provider for guidance.

A blood pregnancy test can detect lower levels of hCG, and will generally show as positive before a urine pregnancy test. In some circumstances, if there is a very high suspicion for pregnancy even though a urine test is negative, or if there is concern for the health of an early pregnancy, your medical provider may recommend a blood test. However, most of the time, a blood test is not needed to confirm pregnancy. 

Trade secret: the urine tests available at your medical provider’s office are just as accurate as the tests at the grocery store! So if you have a positive urine pregnancy test at home, your provider may not need you to come to the office just for another urine test. 

What Pregnancy Tests Cannot Tell You

A pregnancy test is an easy way to determine if someone is pregnant, but you can’t count on it to tell you everything.

Healthy Pregnancy

A positive urine pregnancy test does not give any information about the health of the pregnancy. The body will still make hCG even when there is a miscarriage or problems with the fetus. Additionally, it cannot determine the presence of an ectopic pregnancy, which is when a fertilized egg implants and grows outside of the uterus. 

How Far Along One Is

There is a wide range of what is considered a “normal” hCG level at any given point of pregnancy, so it’s is not a useful marker to determine how far along someone is. A test will also stay positive for a while after a miscarriage or termination.

False Negative

Pregnancy tests may give a false-negative result. This is when it says negative, but pregnancy has actually occurred. The most common reason for a false negative result is that it is too early to detect the amount of hCG needed for a positive test.

False negatives are seen more commonly in urine tests, as blood tests are much more sensitive. If a pregnancy is suspected, taking another test in a few days or contacting a medical provider may be a good idea.

False Positive

A false-positive result can also happen, but this is rarer than a false negative. Sometimes it is a test error or the presence of other medical conditions that can cause the body to make hCG. A medical provider can help determine the reason for a false-positive result.


Once you’ve noticed your period is late, a pregnancy test can be taken to learn whether conception has occurred.There are two types of pregnancy tests: urine and blood. A blood test will show a positive result slightly sooner than a urine test. However, urine tests are generally very accurate and are readily available at many stores. Of course, it’s important to understand what a pregnancy test can and cannot tell you about your body. If you’re experiencing unusual symptoms or suspect something is out of the ordinary, see your medical provider for evaluation

Our Simple Note

We know. It’s pretty incredible that a few drops of urine or one blood draw can tell you about a potentially life-changing event. Whether you’re trying or really not trying to get pregnant, take a deep breath and give yourself a little hug. You’re doing your due diligence by listening to your body and learning everything you need to know about pregnancy tests. The good news is determining a pregnancy early on can allow for access to prenatal care or options and support. So go ahead, be brave. Pee on that stick.

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