After fertilization, the fertilized egg spends a few days dividing as it travels through the fallopian tube to the uterus. Implantation is the moment this embryo attaches to the endometrium (the inner lining of the uterus) — that's when the body starts producing hCG, the hormone detected by pregnancy tests.
How to recognize it (when it's noticeable)
So-called 'implantation bleeding' is usually much lighter than a normal period — generally a light pink or brown discharge for one or two days, sometimes with mild cramping. It's easy to mistake for the start of an early period, which causes a lot of doubt for those trying to conceive.
It's worth repeating: most pregnant people don't notice any physical sign of implantation. Not having this sign doesn't mean anything is wrong.
When to seek an assessment
Heavier bleeding, with strong cramps or lasting several days, isn't typical of implantation and deserves a medical assessment — it may have other causes, including some that need follow-up.