Hello sir, I m 20 years old. My problem is that my sister's period coming late every month like as in previous month her period coming 18 date bt this month her period not come at that date what problem may possible? This problem with her from 2 months, before 2 months her period come at 15 date bt next months her period come 18 date but this month her period not come 15 or 18 date. My sister is 22 year's old. Sir please give me solution.