Yeah macysmama - your damn right, periods are a normal thing to go by with pregnancy (the whole missed periods thing) but to tell you truth they are very unreliable - my son dylan is now 5 days old and was born at 28 weeks - I had periods all the way through... Not very reliable I dont think.
If she wants to know for sure and stop stressing out tell her to get a test. She's probably scared about buying it and doing it (i was) but if you get it for her then maybe it'll help her.
The outcome isn't gonna change if she doesn't do the test, whatever's happened already has and she needs to know to stop worrying.