Heather Corinna replies:
I had unprotected sex with my girlfriend 2 days after her periods was over, as she told me that 10 days after period are safe to have unprotected sex. Her last period was 27th October. Well today, 19th November, she called me and said that her period repeats in 20-25 days and she should have period now but she is not having. Can you tell me the reason? Another question, is that safe to have unprotected sex after period within 10days? Please mail me answer, also fast please.
No, it's not safe for pretty much ALL women.
Sounds to me like your girlfriend has misunderstood how fertility cycles work. While cycles vary between women -- which is part of why the "calendar method" isn't a good one -- a vast majority of women will be most fertile mid-cycle, between around day ten and day fifteen of each cycle, especially when you factor in that sperm can live in the vagina for several days. The chance that is also the highest-risk window for her is very good.
For both of you, understand that if you're going to use natural family planning as a sole birth control method, that involves your girlfriend charting her fertility cycle daily, both by checking her cervical mucus and her basal temperature. With a woman with regular menstrual cycles, after doing this for a few months, she can begin to pretty accurately predict when she is most and least fertile, and then abstain from sex during that window, or only have sex with another method of birth control at those times -- but without doing any of that charting, it's nothing more than a guess and a gamble, because women do not all have the same cycles. So, one woman's safer -- really, no time is "safe" to have unprotected sex if you don't want a pregnancy -- windows may be day one to day nine, and then again from day 20 to 28, while another woman's safer times may be from day one to day four and then again from day 13 to day 25. We're not all the same, so we can't ever talk about "safe days" that apply to all women.
If her periods aren't yet regular, even doing that charting won't be useful. Too, natural family planning of FAM is only generally advised for use as a sole method (the only method used) for couples who are okay with a pregnancy, clear for all STIs and monogamous, and for who CAN do that charting daily. With younger people, I'd say even using full-on FAM with daily charting as the ONLY method of birth control is only something women should be doing if a) they simply cannot afford or access any other method, b) their male partners just won't cooperate with any other birth control, or don't allow them to abstain from intercourse and they can't leave that partner, and/or c) their religious beliefs prevent them from using other methods. Using FAM as a backup with other birth control methods is something I'd advise for younger women, but that's about it.
If her period normally arrives every 20-25 days (which is a very short cycle on the 20-day-end, so I'd be sure she's sure about that), then she's not quite late at this point, but if your last unprotected sex was on the 5th of this month, based on what you're saying here (or earlier), she could certainly take a pregnancy test now and get an accurate result. Since you have had a high risk of pregnancy, I'd suggest she do that now, because if her period does wind up being late, her being pregnant would be a likely reason why.
But from here on out? Use condoms. Seriously. Condoms are cheap, effective, easy to get nearly anywhere on the globe, easy as heck to use, and they really are the easiest and least invasive birth control there is. They require YOUR cooperation, to say the least, but if you're this worried about a pregnancy, that tells me that you cooperating to prevent something you either don't want or are not ready for should be a total non-issue. All you need to do is keep a few handy, and put them on gladly, without ever discussing when she may or may not be fertile.
Here are some links for you: