A woman's cycle begins with the first day of her period. A period can last anywhere from 4-10 days, give or take. About a week after her period ends, ovulation begins. This is the time in which one egg is released from her ovaries. Ovulation is the time in a woman's cycle where she is most likely to become pregnant. Keep in mind that the sperm can live inside the fluid of the vagina for up to a week.
However, since your daughter had sex before her period, unless it was (at most) a week before ovulation, she did it at the time that is safest for a woman to have sex without getting pregnant.
If she was on some kinda of birth control at the time (as every woman should be on two forms of birth control before having sex, to be safe) then the likely-hood of your daughter being pregnant is not very large.
However, with sex there really never is a way to know for sure, especially with someone with an irregular period. Get a pregnancy test as soon as possible and hope for the best. Encourage her to always use protection!
