卫生巾最好两三个小时更换一次，不然容易滋生细菌，除去睡眠时间，最坏的情况是一天更换6次，每月7天，那就是42片，如果加上提前准备的份可能有50片，并不是每个月用一片。
It is best to replace sanitary pads every two or three hours, otherwise it is easy to breed bacteria; apart from sleep time, the worst case is to replace them six times a day, seven days a month, and that is 42 pieces; if you add the ones prepared in advance, there may be 50 pieces, not just one piece every month.