Published on February 24th, 2021
It’s exciting when you and your partner decide it’s time to add another leaf to the family tree. It’s a time to be close, to be excited for the future, and… to start peeing on things—specifically, pregnancy tests.
As you start the baby-making journey, you’ll definitely want to know how soon can a pregnancy test detect pregnancy?
It varies by test, but in short, the soonest a home pregnancy test can read positive is about four days before your first missed period, or about three and a half weeks after an egg is fertilized.
How Pregnancy Tests Work
Pregnancy tests are able to detect a hormone called human chorionic gonadotropin, or HCG. This hormone is produced during the early stages of pregnancy. But understanding that hormone, how pregnancy tests work, and how soon a pregnancy test will read positive may require going back to the basics—the basics of the birds and the bees that is.
Ovulation, Fertilization, and Implantation
While you’re probably familiar with the mechanics of how a baby gets made—AKA the fun part of reproduction—there’s an extraordinary series of events that need to happen inside the mom-to-be’s body in order for a home pregnancy test to display a positive result.
Here’s how that breaks down:
- Ovulation – Many people may mistake an ovulation test as pregnancy test, but they are not the same. Ovulation is a biological process in which a mature egg leaves a woman’s ovary and drops down into the fallopian tube—the tube that connects the ovaries to the womb. During ovulation, your body is at its most fertile. It’s the fertile period when an egg can turn into an embryo, the cellular beginnings of your bundle of joy.
But you’ll need more ingredients than an egg to make a bun in your oven—so to speak.
You’ll also need a little help from sperm.
- Fertilization – To create an embryo, you’ll need two gametes, or cellular reproductive components: the egg, which is a maternal reproductive cell, and sperm, the paternal reproductive cell. When sperm are released into a woman’s vagina through intercourse, they seek out their other half—the egg. But only one sperm can make it all the way through the vaginal canal, into the uterus, and up to the fallopian tube to fertilize the egg. Additionally, an egg is only viable for fertilization for 12-24 hours after ovulation, so timing is critical.
What is fertilization? It’s essentially a microscopic meet-cute. Sperm meets egg, egg meets sperm, sperm fuses with egg to create the start of a whole new person. Aww.
- Implantation – While the fallopian tube is a cozy place for two gametes to meet and fertilize, it’s not the ideal place for them to take the next step of their relationship as a growing embryo. Six to nine days after fertilization, the fertilized egg moves out of the fallopian tube and travels to the uterus, where it implants itself into the uterine wall.
When an embryo is successfully implanted into the uterine wall, the body goes into baby-mode, producing hormones to help the embryo thrive. One of the most important of these hormones is called HCG. The HCG is the hormone pregnancy tests look for to see if you’re pregnant.
The ABC’s of HCG – The Hormone Pregnancy Tests Look For
The human chorionic gonadotropin hormone (mentioned above) is essential to the early stages of pregnancy. It helps the body create a healthy environment for the fertilized egg to thrive.
- Progesterone production –A woman’s HCG level tells the corpus luteum, a gland-like structure that forms after an egg is released during ovulation, to produce progesterone. Progesterone is known as a pregnancy hormone that helps maintain the lining of the uterus for implantation. It also helps prepare your body for labor by softening ligaments and cartilage and loosening your joints.
- Supporting implantation – HCG also stimulates the thyroid gland, which helps the fertilized egg stay implanted.
Because its main role is to support an embryo in the early stages of pregnancy, HCG is produced as soon as a fertilized egg is implanted in the uterine wall. HCG is produced by both the embryo and the uterine wall to help create a nourishing environment for the embryo to grow. After implantation (6-9 days after initial fertilization), your body’s HCG levels increase every 2 or 3 days. HCG can be found in the blood and urine of a pregnant woman.
Pregnancy Test Timeline
After all of that biology, you may still be wondering, how soon will a pregnancy test read positive?
This timeline can help illuminate the timing of ovulation, fertilization, implantation, and when to take a pregnancy test.
- Day 1 – The first day of your period.
- Day 15 – An egg is released from your ovaries through ovulation and is ready for fertilization.
- Day 15 or 16 – Sperm fertilizes the egg.
- Day 22-25 – The fertilized egg travels to the uterus and is implanted in the uterine wall; the body begins producing HCG.
- Day 24-28 – HCG levels increase to detectable levels, a few days before your next period is due.
Pregnancy Tests, Antibodies, and HCG
Approximately four days before your missed period, you can take a test to see if that happy meeting between sperm and egg has taken place.
But how does that little test strip work?
A simple line pregnancy test is made up of four essential parts:
- The Sample Pad – This is the part of the pregnancy test that absorbs your urine—aka, the part you pee on.
- Antibodies – Monoclonal antibodies are embedded in the test strip, right next to the sample pad. Monoclonal antibodies are lab-made proteins designed to interact with cells in certain ways. In a pregnancy test, monoclonal antibodies are built to bind with HCG collected in the sample pad. These antibodies are also attached to dye.
- Test Line – When HCG binds with the dye-infused monoclonal antibodies, they will react and form a line to indicate a positive pregnancy test. If there is no HCG in the urine (that is, if there is no pregnancy), there’s nothing for the dyed monoclonal antibodies to bind to, and no line will form.
- Control Window – The empty window at the top of the pregnancy test is designed to show you whether the test itself is functioning. The dyed antibodies that are not attached to HCG will move into the control area where they’ll find—you’ll never guess—more antibodies!
The control window antibodies are designed to recognize the dyed antibodies and bind to them. This causes the dye to create a “control line,” proving that the antibodies are active and the test is working as it should.
So in this test, two lines make a positive pregnancy test while one line makes a negative.
How accurate are pregnancy tests?
Many pregnancy tests claim to be 99% accurate. But do you ever wonder why you never see a test that claims to be 100% accurate? That 1% allows for human error, extraordinary circumstances, and variables you can’t always control.
Factors that Cause a False Negative
Can you get a false negative pregnancy test? Absolutely. But you can decrease your risk of getting a false negative by keeping an eye out for the variables that cause a negative pregnancy test result, such as:
- Urine concentration – The more diluted your urine, the harder it is for a pregnancy test to detect accurate amounts of HCG (so chugging water before your pregnancy test is not the way to go). Instead, try to take your pregnancy test first thing in the morning when the chemicals in your urine are at their most concentrated.
- Testing too early – If you’re trying to get pregnant, you may be meticulously tracking your menstrual cycles so you can pinpoint your most fertile periods. But not every woman operates on a 28-day cycle. If you get a negative test result, you might have caught your egg right at implantation, before HCG levels are high enough to be detected. Try waiting a few days and test again to ensure an accurate reading.
- Delayed implantation – Typically, a fertilized egg will mosey down into the uterus for implantation 6-9 days after fertilization. But that’s not always the case for every woman. In fact, for 10 percent of pregnant women, the embryo doesn’t implant until the first day of your missed period. And remember, HCG isn’t produced until implantation, so you may not get a positive pregnancy test until week 6 or 8 of your pregnancy!
Factors that Cause a False Positive
A false positive pregnancy test result is extremely rare. The monoclonal antibodies are designed to interact with HCG, so the chances of getting a positive result when you aren’t pregnant happens very, very infrequently.
Some possible factors that could lead to a false positive include:
- Ectopic pregnancy
- Chemical pregnancy
- Recent miscarriage
- Reading a test’s evaporation lines instead of the test lines
- Medications such as
- Certain fertility medications like Novarel, Pregnyl, Ovidrel, or Profasi
- Anti-anxiety medications
- Parkinson’s disease medications
- Diuretics like furosemide
- Medical conditions (though this happens rarely) such as:
- Urinary tract infections
- Kidney disease
- Ovarian cysts (specifically corpus luteum cysts)
- Ovarian cancer
When You’re Positively Pregnant, Find More Answers with SneakPeek!
When that test strip finally tells you the good news, you may feel completely overjoyed—then overwhelmed with more questions.
One of the biggest questions in a parent’s mind after a positive pregnancy test?
Am I having a little boy or a little girl?
You won’t have to wait long to find out with a little help from the SneakPeek Early Gender DNA Test. At 99.9% accurate, this gender prediction test can tell you whether you’re having a boy or a girl as early as eight weeks into pregnancy.
Even better? There are no vague test strip results to interpret, no sticks to pee on—just answers. Find yours with the SneakPeek Early Gender DNA Test!
UTSouthwestern Medical Center. How early can home pregnancy tests show positive results? https://utswmed.org/medblog/home-pregnancy-tests/
Healthline Parenthood. When You Should Take a Pregnancy Test. https://www.healthline.com/health/pregnancy/five-signs-to-take-pregnancy-test
Britannica. Fertilization. https://www.britannica.com/science/fertilization-reproduction
Healthline. What is Ovulation? Cycle Timeline, Pain and Other Symptoms. https://www.healthline.com/health/womens-health/what-is-ovulation
University of Michigan. Human Chorionic Gonadotropin. https://www.uofmhealth.org/health-library/hw42062
What to Expect. Your Guide to Pregnancy Hormones. https://www.whattoexpect.com/pregnancy/pregnancy-health/pregnancy-hormones.aspx#progesterone
Mayo Clinic. Home pregnancy tests: Can you trust the results?