I had unprotected sex 3 days ago & I was wondering when does pregnancy start ? Even if you don't have any symptoms .

It starts when an egg becomes fertilized and implants in the uterus. The pregnancy may not continue for various reasons, but that is when it starts.

Pregnancies are dated from the first day of the last menstrual period simply because few women will know the exact day when they conceive (which often isn't the day they had sex). Ultrasounds done early in pregnancy can give a bit better idea of when conception likely occurred by taking measurements of baby.

Having unprotected sex three days ago may result in pregnancy, but only if you ovulated within a few days of having had the sex and only if all other factors required were in place.