On April 30 th she must keep her veset , and refrain from relations that day. She does an internal check with an examination cloth and looks for any drop of blood. She should also calculate how much time has passed between her last two periods. She should count twenty-eight more days, which would bring her to May 25 th , and on that day she should refrain from relations. The next time, she counts the interval between her last two periods.
Most women wind up keeping their veset on both the thirtieth and thirty-first days.
If your cycle is more erratic than that, see if your doctor can help. With your Rabbi's guidance, it may be possible to correct it without violating Jewish Law. Keeping the Laws of Niddah properly are very likely to help you get that extra merit from Hashem that will help you conceive a child. And best of all, you will then know that he or she was conceived in the best of situations. It is also good to say each Friday evening, as you light Shabbos candles, the special prayer that women say when they light the Shabbos candles.