Whenever Elizabeth Smart started a period is irrelevant to the discussion. The only question is whether or not she could have conceived and birthed a child in the time period while she was being held hostage.
The answer is "Yes!" it is possible, because the human gestation period is 38 weeks or 8.85 months.
A random definition of "pregnancy", by American Doctors, does not change the fact that gestation begins at conception and ends at birthing.
And by the way, 40 weeks are not 10 months. If a month were to consist of 4 weeks, then there would be 13 months in the year. 52/4 = 13.
There is absolutely no reason to believe Elizabeth Smart had a baby. FOR THE RECORD.