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Heather Corinna replies:
For a majority of women, the time period you're talking about, a week after your menstrual period ends, is a window of the highest fertility: in other words, the time when most women would be at their highest pregnancy risk.
Thing is, with any question like this, we're always going to have two equally true answers:
1) It depends on YOUR individual fertility cycle, and
2) ANY time a woman has unprotected genital sex with a cisgender male partner -- read: direct penis-to-vulva contact without a reliable method of birth control -- she has a real risk of becoming pregnant.
So, let's chat a little about fertility cycles and what the heck I'm talking about.
If you take a look at this piece, you can get a good idea of how the fertility and menstrual cycle works. There are times in every woman's cycle when she is more or less likely to become pregnant and what that depends on is when that particular woman ovulates; when she is near or at the end of her proliferative, or follicular, phase and in her ovulatory phase. That is then when the ovary ruptures and an egg is released, which can be fertilized by sperm to create a pregnancy. For the "average" woman with an average cycle of 28 days, that day will be on day 14 (the day a woman's period begins is day 1).
The window after that egg is released for it to be fertilized is about one day, however, you've got to remember that sperm can survive in the vagina for anywhere from three to seven days. So, if that average woman had intercourse where her male partner ejaculated within her anywhere from Day 7 to day 16, she would have a high risk of pregnancy. She may also have risks on other days in her cycle, but for that woman, intercourse in that time period would be the time of her highest risks.
How does someone know when they ovulate? The only way to have an accurate idea about how one's own fertility cycle works is to chart it, every day, through both examining cervical mucus and by taking basal temperatures. Every day. When we do that daily, we can see over each cycle, when we have the kind of mucus women do when most fertile and when we have the kind of mucus where we're least fertile, as well as seeing the temperature changes that happen during the different times of our cycles. If we have a regular cycle, after several months of doing this, we can become able to predict with a decent amount of accuracy when, in the next cycle, we will probably be most and least fertile.
Some women may be like our average woman above. Others may ovulate on day 11, some on day 16. And far less frequently, some women may even ovulate more than once in a given cycle. To be clear: all women are not the "average" woman, so we can't know when we're most fertile just by counting days (which is sometimes called "the rhythm method").
Toni Weschler, the awesome author of both Taking Charge of Your Fertility -- which is basically the bible when it comes to fertility charting -- and Cycle Savvy, a book about cycles for teens (and she does not recommend FAM for teen women) explains more here. And if you or your partner really want to know more about charting and fertility, it'd be a good idea to get your hands on those books. Once a woman gets the hang of charting, it's not all that complicated, but at first, it really is detailed and involved enough that it's something one needs a good book for.
Without doing all of that daily homework with charting, we have no way of knowing when that happens, save after the fact. As well, using charting as a method of birth control -- often called Natural Family Planning or the Fertility Awareness Method (FAM) -- is only recommended for a) women whose cycles have regulated, which often is not the case with teen women b) couples who are ready for a potential pregnancy, c) couples where the male partner is in real cooperation with the female partner in abstaining from genital-to-genital sex in and around the fertile window (or where a backup method of birth control will be used then) and d) couples who have practiced all the parts of safer sex -- latex barriers, monogamy and screenings, for at least six months before going without barriers -- to know their STI (sexually transmitted infection) risks are minimal. Generally, FAM is only a good choice for women with all three of those factors accounted for, and tends to be best for women who know they can and will chart without fail, who have problems with, objections to, or lack of access to other reliable methods of birth control, or who are using charting in conjunction with another sound method of birth control.
It's likely obvious at this point that for teens, this isn't a sound method, and just guessing and gambling isn't a sound method for ANYONE. If you or your partners do NOT wish to become pregnant, you need to only be having genital-to-genital sex IF using a reliable method of birth control which you can use properly. Charting probably isn't it, but that's not a big deal since there are a LOT of reliable methods out there, most of which are very easy to use -- and involve less effort than charting does -- and are affordable.
Of course, the other big issue here is that FAM doesn't protect anyone from sexually transmitted infections, which are really a bigger risk, especially for young people, than pregnancy is. Around one out of every four young adults between the ages of 15 and 22 has an STI (around one of every ten sexually active teen women in the U.S. will become pregnant). Teens and young adults are the fastest growing group of people with sexually transmitted infections. The bonus is that the method (sparing not having genital sex at all) that protects everyone from STIs best is also a reliable, inexpensive and easy-to-use method of birth control too: the condom.
When any person is really ready for partnered sex that poses STI and/or pregnancy risks, they also have to be really ready to deal with safer sex and birth control. If you or your partners are not, then the very best thing you can do is just to wait until you are. Not only is that best for your physical health, it's best for your emotional well-being. Plus, sex can only be so fun when you're freaking out before, after and/or during it because you know you're taking risks you shouldn't be. What a drag!
If you need help working out reliable birth control methods and safer sex, you can talk to your doctor, gynecologist or to any family planning clinic, like Planned Parenthood. Any of those folks can help you find out what methods are best for you and yours, show you how to use them, and help you obtain them.
Here are a few more links to help you out: