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A woman who has the custom of keeping the Or Zarua only needs to keep the additional onah for the vesatot based on the last period, and not for any veset that is carried over from previous months.
There is dispute about how a yom hachodesh veset carries over. The yom hachodesh is uprooted once the date passes and you did not get your period on that date. Should you get your period before that date, you still must observe that onah that month. For example, if a woman got her period on 17 Tishrei, and then again on 4 Cheshvan, she still must observe 17 Cheshvan as yom hachodesh (as well as 4 Kislev). If 17 Cheshvan passes and she does not get a new period, that veset is uprooted and she does not observe it in the future.
If her next period comes on 2 Kislev, and she still experiences bleeding on 4 Kislev (her yom hachodesh), there is dispute among poskim as to whether that date is uprooted. Some poskim hold that the onah cannot be uprooted since there was a blood flow on that day. She would then carry that date over to the next month. Others hold that, since we only calculate from the onset of the flow and not from the continuation, that date is uprooted. The position of this site is the latter: the onah is uprooted if she did not begin menstruating on that date. You should ask your rabbi which position to follow. If you don't have a rabbi to ask, you may follow the position of this site.
Please feel free to get back to us with any further questions.