Home pregnancy tests detect a hormone called hCG (human chorionic gonadotropin) in your urine. Your body starts producing hCG shortly after a fertilized egg implants in the uterine wall, and the hormone roughly doubles every day during the first week. The test strip uses a simple but clever antibody-based system to catch that hormone and turn it into a visible line, usually within minutes.
What Happens Inside the Test Strip
A pregnancy test strip has four main sections laid out in a row: a sample pad where you apply urine, a conjugate pad loaded with antibodies, a nitrocellulose membrane with two detection lines, and an absorbent pad at the far end that pulls liquid through like a wick.
When urine hits the sample pad, it flows forward into the conjugate pad. Here, tiny particles (often made of colloidal gold, which appears pink or red) are coated with antibodies designed to latch onto hCG. If hCG is present, these particles grab onto the hormone and travel together down the strip. If there’s no hCG, the particles keep moving but have nothing attached.
Next comes the test line. This is a thin stripe of different antibodies locked in place on the membrane. These antibodies also recognize hCG, but they grab a different part of the molecule. So the hCG gets sandwiched between two antibodies: one on the gold particle, one fixed to the membrane. That anchors the colored particles in a visible line. No hCG means no sandwich, no line.
Farther down the strip is the control line. This line captures the gold-particle antibodies themselves, regardless of whether hCG is present. It always appears if the test worked correctly. One line means not pregnant. Two lines means pregnant. No lines means the test malfunctioned.
The Hormone Behind the Result
hCG enters your bloodstream and urine after implantation, which typically happens 6 to 12 days after ovulation. In a study tracking 142 pregnancies, researchers measured hCG in first-morning urine each day following implantation. On day one, levels were barely detectable. By day four, concentrations had multiplied roughly 18-fold. By day seven, they were more than 100 times higher than day one. This rapid doubling is why waiting even a day or two can turn a negative result into a clear positive.
Most home pregnancy tests can detect hCG at concentrations between 20 and 50 mIU/mL, though some early-detection tests claim sensitivity as low as 6.3 to 12.5 mIU/mL. Blood tests at a lab are more sensitive still, picking up levels as low as 1 to 2 mIU/mL. For most people, urine hCG crosses the home-test threshold around four weeks after conception, which lines up with the first day of a missed period.
Why Timing and Hydration Matter
hCG concentration in urine fluctuates throughout the day. First-morning urine is the most concentrated because it’s been accumulating in your bladder overnight. If you test at another time, aim for urine that’s been in your bladder for at least three hours. Drinking large amounts of water beforehand can dilute hCG below the test’s detection threshold and produce a false negative, especially in the earliest days of pregnancy when levels are still low.
Testing too early is the most common reason for a negative result that later turns positive. If you test before your missed period and get a negative, your hCG simply hasn’t risen enough yet. Retesting a few days later often gives a definitive answer.
Faint Lines and Evaporation Lines
A faint line that has the same color as the control line (pink on pink tests, blue on blue tests) is a positive result. It just means hCG levels are on the low side, which is normal in very early pregnancy. Any colored line, no matter how faint, counts.
An evaporation line is different. It appears when urine dries on the strip after the reading window has passed, usually beyond 10 minutes. Evaporation lines are colorless: gray, white, or shadow-like rather than pink or blue. They also tend to be thinner than the control line and may not stretch fully across the window. The simplest way to avoid confusion is to read your result within the timeframe printed on the box, then discard the test. If you’re unsure, test again the next morning with a fresh strip.
How Digital Tests Differ
Digital pregnancy tests use the exact same antibody strip inside the plastic housing. The difference is that instead of you reading the lines, a small optical sensor inside the device reads the line intensity and displays “Pregnant” or “Not Pregnant” on a screen. This eliminates the guesswork around faint lines and evaporation marks. The underlying chemistry is identical, so digital tests aren’t more accurate. They’re just easier to interpret.
What Can Cause a False Positive
False positives on home pregnancy tests are uncommon, but they do happen. The most straightforward cause is a fertility medication that contains hCG itself, since the test can’t distinguish between hCG you injected and hCG your body produced. Certain other medications have also been linked to false positives, including some antipsychotics, anti-seizure drugs, and specific anti-nausea medications.
A chemical pregnancy, where a fertilized egg implants briefly but doesn’t develop, can also produce a true positive that’s followed by a period. The test correctly detected hCG; the pregnancy simply didn’t continue. In rare cases, certain cancers that produce hCG can trigger a positive result as well. An expired or improperly stored test is another possibility, though this is more likely to give an invalid result than a clear false positive.
What Can Cause a False Negative
Besides testing too early or with diluted urine, there’s an unusual phenomenon called the hook effect. At extremely high hCG levels, typically above 500,000 mIU/mL, the test strip gets overwhelmed. So much hCG floods the strip that it saturates both sets of antibodies independently, preventing them from forming the sandwich that creates a visible line. This is rare and almost exclusively occurs with molar pregnancies or other conditions that produce extraordinarily high hCG. For the vast majority of pregnancies, this isn’t a concern.
Using an expired test, not following the instructions (like not holding the strip in urine long enough), or reading the result too early can also lead to false negatives. If you get a negative result but still suspect pregnancy, testing again in two to three days gives hCG levels time to rise into a clearly detectable range.