does she have "regular" menstrual period? if so when was her last period if its about 5 days of less, chances of her getting pregnant is quite low. for "safe" days with women with regular menstrual visit its about 5 days before and 5 days after periods.
now if she gets "irregular" menstruation its harder to say, because the "safe" days would only mean the after menstruation days (the 5 days after)
thats what i recall with that natural birth control method. now regarding your case.. its probably 50/50 or higher, good luck bud!
