And yet my slow realization, after being asked (as resident student nurse/health nut person ‘round here) numerous times about “can I get pregnant if…” is that most, or at least many girls, do not know this stuff. Which means it was never taught. Which means that someone, somewhere, decided that this was not relevant information to teach adolescent and adult women.
But knowledge is power, so here goes.
(last minute note: this is long ‘n stuff, but I decided to post it anyway because I really think it is important. I fervently hope you either a) already know this, or b) take the time to learn a little more right now).
This is how to determine your fertility window. To start off, your fertility window is the time surrounding ovulation, which means it is the window in which you are most likely to get pregnant.*
1. Your ovulation cycle is controlled by your hormones (estrogen and progestin, mostly), and it’s the rise and fall of these hormones that cause you to ovulate and then later menstruate. They also cause all the in-between stages that we usually ignore (but are also important.) This is why birth control pills work (but we’ll get there in a moment). For now, suffice to say that spikes and dives in different hormones trigger different reactions within your body.
2. Most women’s cycles last 28-32 days. If you know how long your cycle is, and you’re consistent, it’s even easier to track your fertility window.
3. In the cycle’s of most women, ovulation often occurs between day 11 and day 21. This means that you ovulate 11-21 days after the START of your last period (NOT the end!). To find a more accurate ovulation day, track your period for a few months. Ovulation day is 14 days before your last period (the time from start of period to ovulation can vary, but the 14 days after ovulation is usually consistent). From there, look at how many days you typically have between the start of period-before and your designated ovulation. This should give you a good idea of your personal ovulation day, or at least give you a smaller range (and it’s usually 11-21 days out).
4. But wait! There is more. If you’re having sex, you should know that sperm can survive within the cervical/uterine environment for up to 5 days. This means that if you have sex 5 days before your ovulation day, you can still possibly get pregnant. So to find the beginning of your fertile window, subtract 5 from your ovulation day (we’ll go with the data and say 11-5 is day 6).
5. Now, an unfertilized egg is only alive for 12-24 hours after it is released from the ovary. This means that you can get pregnant up to one day after your ovulation day. Therefore, you need to add one more day onto your ovulation day to find the end of your fertile window. To estimate with our 11-21 day range, that’s day 22.
6. So to conclude, most women’s fertile windows will fall somewhere from day 8 to day 22 after their last period began. That does not mean you will, or even can, get pregnant every day in this range, but that you will be fertile at some point. It does looks like a big range of days. But if you track your own periods as stated in step 2, you may be able to discern your body’s personal trends in ways that allow you to narrow your personal window. If you aren’t having unprotected sex within that window, the chance of pregnancy is much less.**
This is the basis of natural family planning, by the way. But even if that’s not your preferred method of birth control (there are many ways which are more effective; this one is about on par with the withdrawal method— about 24 out of 100 women who use this with some error will become pregnant over the course of a year) it’s still an important thing to know about your body.
A note about being on the pill: on most birth control pills, you are taking supplemental hormones that prevent your body from ever ovulating in the first place. So as someone once asked me, if you’re on the pill you are not at any real risk of getting pregnant from having period sex. It’s highly unlikely that you would get pregnant from period sex regardless of being on birth control, but if you’re on the pill there are no eggs involved. You are not ovulating. If you miss a pill or two or three, you need to look at all that paperwork that came with your medications to see if you are still covered. In some prescriptions, like mine, if you’re late on one day it doesn’t cause enough hormonal fluctuation to catalyze ovulation. But other prescriptions differ, so checking the paperwork is a good idea.
* This is the part where I point out that every girl and every girl’s body is miraculous and different. There are plenty of crazy pregnancy stories. Caution and discretion and protection are all very, very good things to use, at least in my book. Also, condoms are great and important because hi! STIs are nasty things! But say your condom breaks— because that shit happens. How bad should you be freaking out? Are you DeFiNiTeLy pregnant? This formula could help you to figure out your next step.
** This is the second part where I reiterate that there are plenty of “we never saw it coming” pregnancy stories out there, and that I believe everyone should find a method of birth control that is affordable, effective, and that they are comfortable with using every time they have sex.
Know your body! Because your body is amazing, and every girl should know at least a little bit about how it works. And finally, Planned Parenthood is and always has been an incredible resource for learning about this stuff.
This information is not meant as a substitute for professional advice.