In general, yes. While it is possible to have heavy bleeding while pregnant, this is not normal and likely means miscarriage. If you were that late as well it can be expected your cycle will be heavier than usual too.
so if the first day is really heavy, it is already considered a period?
If you get a period, you're not pregnant. A period is the uterus lining shedding to get rid of an unfertilized egg, if you are pregnant, your egg is fertilized, therefore the lining will not shed. You can have pregnancy related bleeding though, and it may occur around the time a period would, so if it's a different flow/length than normal, it's possible it's pregnancy related.