If sexual intercourse occurs during the fertile period of a woman's menstrual cycle (around ovulation), conception (fertilization of the egg-cell) will occur in the first 12 hours after ovulation. Nevertheless, we do not call this phase "pregnancy". Pregnancy occurs when the fertilized egg-cell embeds itself in the uterus mucosa (implantation). Implantation happens 8-10 days after conception. After implantation, a pregnancy test become positive.
Spermatozoids can remain alive in a woman’s oviducts for 48 hours at most (although some expert claim up to 7 days!). Therefore, it is safe to say that conception occurs, at most, 48 hours after sex.