Help and pls answer this

having sex one day before your menstrual cycle starts is safe so as to avoid a prenancy?can pregnancy be even caused after 24 to 48hrs after ovulation?

It is never safe to have sex without protection. don't confuse ovulation with your period, ovulation happens about two weeks before you get your period.
Ovulation is the release of a single, mature egg from the ovarian follicle. The human ovary produces a multitude of ova during the course of a month, largest of which is expelled into the pelvic cavity and swept into the Fallopian tube. Ovulation does not follow a regular pattern between ovaries each cycle and which ovary releases the egg is fairly arbitrary. Once released, the egg is capable of being fertilized for 12 to 24 hours before it begins to disintegrate. If the released ovum or egg is fertilized and successfully implants, it results in pregnancy.
If the egg is not fertilized, it is passed from the reproductive tract during menstrual bleeding, which starts about two weeks after ovulation. Occasionally, cycles occur in which an egg is not released; these are called anovulatory cycles. Note that you can get your period even though you are not ovulating. Your ability to ovulate can vary from month to month.

here you will find information about ovulation and sex: http://www.babyhopes.com/articles/afterovulation.html

theres a low chance .
but of course its possible.