A pregnancy test measures a hormone called human chorionic gonadotropin, or hCG, in your urine or blood. This hormone is only produced after a fertilized egg implants in the uterus, making it a reliable signal that a pregnancy has begun. Even the simplest drugstore test works by detecting whether hCG has crossed a specific concentration threshold.
What hCG Does in Early Pregnancy
Once a fertilized egg attaches to the uterine lining, a layer of cells in the developing placenta begins producing hCG and releasing it into the bloodstream. About 99% of this hormone enters the mother’s blood, with smaller amounts filtering into urine.
The hormone’s main job in early pregnancy is keeping the ovaries producing progesterone and estrogen. Normally, the structure in the ovary that releases an egg each month (called the corpus luteum) breaks down after about two weeks. hCG prevents that breakdown, essentially telling the ovary to keep supplying the hormones that maintain the uterine lining and support the embryo. This continues for roughly the first six weeks, until the placenta grows large enough to take over hormone production on its own.
How Home Pregnancy Tests Detect hCG
Home tests use antibodies embedded on a test strip that bind to hCG molecules in your urine. When enough hCG is present, it gets “sandwiched” between two antibodies, triggering a visible line, plus sign, or digital readout. If hCG is below the test’s detection threshold, no signal appears and the result reads negative.
Not all home tests are equally sensitive. The most sensitive widely available test, First Response Early Result, can detect hCG at concentrations as low as 6.3 mIU/mL, which is enough to identify over 95% of pregnancies by the day of a missed period. Other early-detection brands pick up hCG at around 25 mIU/mL, catching roughly 80% of pregnancies at that same point. Many standard tests require 100 mIU/mL or more, which means they detect only about 16% of pregnancies on the day of a missed period, though accuracy improves quickly in the days that follow as hCG levels rise.
When hCG Becomes Detectable
Your body doesn’t start making hCG until implantation occurs, and implantation doesn’t happen the moment of conception. After ovulation and fertilization, the embryo travels down the fallopian tube and embeds in the uterine wall, a process that typically takes six to twelve days. Trace levels of hCG can appear in blood as early as eight days after ovulation, but for most people, levels don’t climb high enough for a home urine test to pick up until around the time of a missed period, or one to two days before if you’re using a high-sensitivity test.
hCG roughly doubles every 48 to 72 hours in early pregnancy. This rapid rise is why waiting even two or three extra days after a negative result and retesting often produces a clearer answer. Testing too early is the most common reason for a negative result in someone who is actually pregnant.
Blood Tests: Qualitative vs. Quantitative
Your doctor can order two types of blood-based hCG tests. A qualitative blood test simply reports whether hCG is present, giving you a yes-or-no answer similar to a home test but with greater sensitivity. A quantitative blood test measures the exact concentration of hCG in your blood, reported as a specific number in mIU/mL.
Quantitative tests are useful beyond just confirming pregnancy. Doctors use them to track whether hCG is rising at the expected rate, which helps assess whether a pregnancy is progressing normally. They’re also used to monitor for ectopic pregnancy, miscarriage, or certain complications where hCG levels plateau or drop when they shouldn’t.
What Can Cause a False Positive
A positive test without an actual pregnancy is uncommon, but it happens. The most straightforward cause is an hCG-containing injection used during fertility treatment. If you’ve recently had a trigger shot to stimulate ovulation, residual hCG from the injection can produce a positive result for up to two weeks afterward.
Certain medical conditions also produce real (not false) hCG that registers on tests. People going through menopause can have low levels of hCG produced by the pituitary gland, typically too low to trigger a home test but sometimes enough to show up on a sensitive blood draw. Chronic kidney disease can cause hCG to accumulate in the blood because the kidneys aren’t filtering it out efficiently, though levels usually stay below 25 to 100 international units per liter. Rarely, some cancers of the breast, lung, gastrointestinal tract, or reproductive organs produce hCG as a byproduct.
True assay interference, where a test incorrectly reads positive despite no hCG being present at all, is estimated to occur in roughly 1 in 1,000 to 1 in 10,000 blood tests. This is sometimes called “phantom hCG” and is caused by antibodies in the blood that interfere with the test chemistry.
What Can Cause a False Negative
The most common reason for a false negative is simply testing too early, before hCG has risen to detectable levels. But there’s a lesser-known cause that can produce a false negative even late in pregnancy: the hook effect. When hCG concentrations become extremely high, the excess hormone can overwhelm the test strip, saturating both antibodies and preventing them from forming the “sandwich” needed to generate a signal. In standard home tests, this generally doesn’t occur until hCG reaches around 1,000,000 mIU/mL, a level seen in rare conditions like gestational trophoblastic disease. But a related phenomenon, sometimes called the “hook-like effect,” can occasionally cause false negatives in normal pregnancies at lower concentrations when the test’s antibodies don’t recognize certain hCG variants that become more prevalent later in pregnancy.
Dilute urine can also lower your chances of getting an accurate result. hCG concentration is highest in the first urine of the morning, which is why most test instructions recommend testing at that time, especially if you’re testing early.
Why It’s All About Timing
The accuracy of any pregnancy test comes down to whether enough hCG has accumulated to cross the test’s detection threshold. A blood test can pick up pregnancy slightly earlier than a urine test, and a high-sensitivity home test can detect it days before a standard one. But all of them are measuring the same molecule. If you get a negative result and your period still hasn’t arrived a few days later, retesting with first-morning urine gives hCG levels time to climb into a clearly detectable range.