think of it this way.
day 1 - 28.
day 1 - 5 -------> menstruation/mens/regla
day 6-8 ---------> safe
day 9 -----------> medyo medyo
day 10 - 14 -----> FERTILE
day 15 -17 ------> medyo medyo
day 18 - 28 ------> umaariba na
isipin mo nalang, lahat ng babae (na hindi baog), madaling mabuntis. there's no such thing as "safe" sex on "safe" days, and am speaking in terms of the woman's ability to get pregnant. because trust me, no matter how you put it, any woman who have mens can always get pregnant, anytime. (excluded are the women who are taking contraceptives, sterile)
so there. think about it once, twice, thrice. there's nothing more scary than waiting for your girl's first day mens, especially if you both know she's delayed.