To have an actual period would require a complete shedding of the uterine lining, something that CANNOT happen while maintaining a pregnancy. Many women (as demonstrated on any pregnancy forum) can have unexplained spotting that can be mistaken for a period. Some women will have this spotting throughout their first trimester, but it is NOT a period. Again, an actual period involves the complete shedding of the uterine lining.
Andi
Yeah as Andi said even those women who said they have had like normal flow etc its not a period as in a true period where ur egg wasnt fertilised its just a bleeding which I dont think drs can really explain.
Early pregnancy bleeding
Bleeding in early pregnancy is a common occurrence, but this does not signal a true menstrual cycle, as pregnancy hormones prevent the ovary's release of an egg as well as the buildup and subsequent shedding of the uterine lining. If these episodes are intermittent, they can be perceived as periods, and the mother-to-be may not recognize that she is pregnant.
---from http://www.drspock.com/article/0,1510,6004,00.html
Brittany