Implantation bleeding occurs during the earliest stage of pregnancy, typically 6–12 days after ovulation (or conception), when a fertilized egg embeds itself into the uterine lining. At this point, you are pregnant, but only by a few days—far too early for any standard pregnancy test to detect hCG (human chorionic gonadotropin), the hormone pregnancy tests measure.
The bleeding happens because the implanting embryo creates small blood vessels and breaks tiny capillaries in the endometrium as it burrows in. This causes light spotting or bleeding that lasts just a few hours to a few days, typically lighter and shorter than a period. The bleeding is often pale pink or brownish rather than bright red, because it's older blood.
Importantly, not all pregnancies produce implantation bleeding—many pregnant people never experience it. Additionally, light vaginal bleeding early in a cycle can come from other causes like hormonal fluctuations, infection, or cervical irritation, so implantation bleeding isn't a reliable pregnancy indicator on its own. A standard urine pregnancy test won't show a positive result until 10–14 days after ovulation, when hCG levels are high enough to detect. For absolute confirmation at any stage, a blood test measuring hCG or an ultrasound provides definitive evidence of pregnancy.