A pregnancy test detects pregnancy hormone, human chorionic gonadotropin (hCG), in your urine. Some tests also detect a variation of this hormone, known as hyperglycosylated hCG (H-hCG). The regular hCG is produced only after an embryo implants in to the endometrium, while H-hCG begins to be released earlier, shortly after fertilization has occurred.
When pregnancy tests tell you how much hormone they detect, they usually tell you the minimum amount of hCG the test requires in order to detect this hormone. The level of H-hCG is usually higher than that of hCG, and most pregnancy tests are not sensitive to the hormone H-hCG.
If a pregnancy test does detect H-hCG, you’re more likely to get a positive result early. If a pregnancy test is not sensitive to H-hCG, and only detects regular hCG, getting an early positive result is less likely. The great majority of pregnancy tests on the market are not accurate at detecting H-hCG.
Early results tests, the ones that claim to give results three or four days before missing the period, assume a 14-day luteal phase, which is the time between ovulation and when you get your period. If your luteal phase is usually 12 days, four days before your missed period would be 9 days after ovulation, which is too early to test. If you have a luteal phase of 15 days, however, 4 days before your missed period is 12 days after ovulation. Your hormone levels may still not be elevated enough to be detected that early, but you’ve got a much better chance than someone with a shorter luteal phase.
On the insert of home pregnancy tests, it says that there is 99% accuracy on the day of your missed period — and not for early results. Research studies that compared how much hCG the test claimed to detect and how much it actually detected, the tests were only 46% to 89% accurate.
Another thing to keep in mind is whether you know if your period is actually late.
Charting your basal body temperature and to figure out how long your average luteal phase is necessary to be sure. Otherwise, it’s only an estimation. According to the FDA’s website, 10 to 20 out of 100 women will not get a positive pregnancy test result on the day they think is just after their missed period, even if they are pregnant.