F you have sex several hours before period can sperm survive the period and cause pregnancy with an early ovulation after period?

Life of sperm: Sperm have a practical life span of 48-72 hours, so having sex prior to a period which is generally fourteen days before your next ovulation will not result in pregnancy.

Depends: Pregnancy has been documented from exposure on any day of a woman's cycle, including during menses. It is rare, but some women ovulate at odd intervals & those with irregular cycles would be most at risk. If pregnancy is really something you need to avoid, you should always use BC or consider EC.