Pregnancy does not happen immediately after sex. Fertilization may occur soon after intercourse if ovulation has already happened, or it may occur several days later because sperm can survive in the reproductive tract while waiting for an egg to be released.
The fertile window is generally the six days ending on the day of ovulation, with the highest chance of conception when sex occurs during the two days before ovulation. After fertilization, implantation usually begins about 5 to 7 days later. This is when the developing pregnancy attaches to the uterus and pregnancy hormone production begins.
A pregnancy test may not become positive right away. Blood tests can sometimes detect the pregnancy hormone around 8 to 10 days after ovulation, but testing too early can still give a negative result even when pregnancy has occurred.
Not every act of sex during the fertile window results in pregnancy, and it can take several menstrual cycles to conceive. If pregnancy has not occurred after 12 months of regular unprotected sex, or after 6 months for someone age 35 or older, a fertility evaluation with an OB-GYN or fertility specialist is recommended.