I am sorry to hear about your trails and tribulations. With the information you give, your fiancé is most likely the baby's father.
You do Not say if you took emergency contraception after the rape. So we have to assume that March 18 was a normal period. Do you have any reason to think that it was not?
Emergency contraceptives are not 100% effective, only about 87%.
Remember not to freak out when you see the gestational age of your baby on the scan. Day 1 of the gestational age is 2 weeks BEFORE conception.