Trileptal side effect in women?

I want to know, does trileptal normally effect a woman's menstrual cycle? I've read on some med sites that it shouldn't but I want to know for sure... My sister is about 19 yrs old, normal cycle. Started weening onto trileptal and off of keppra over a month ago, taking about four weeks to complete. Her cycle originally 28 days before going on either med and went up to 30 after she started keppra. It has been 34 days now since her last period. At least when she went on keppra she would still get pms symptoms when they would usually be expected, letting her know it was arriving soon, but that is not the case now.. no syptoms, no period, no sign of period. She is starting to wonder if this is something she should worry about... Does any one have any information that might help?