How can a woman get her period while pregnant?

She cannot: During a normal menstrual period, a woman's uterus sheds (loses) its lining, leading to the bleeding... and she gets a new lining the next cycle. If she were pregnant, and replaced the lining of her uterus, the pregnancy would be lost too. Pregnant women do get vaginal bleeding sometimes, but it is not a menstrual period (even if it sort of looks like one).