If you’re like many women, you probably have a love-hate relationship with your period. Trying to figure out when it will come, how long it will last, and if you can get pregnant at this time or that during your cycle can feel like a full-time job — one that requires a degree in biology, no less! But all you really want is to be in charge of when (or if) you become a parent.

If you ovulate regularly (not every woman does), you have a monthly "fertile window" when you’re most able to get pregnant. This fertile window varies from woman to woman and sometimes also — sigh — from month to month.

This can make it hard to know when you’re at your most fertile, which usually — but not always — occurs mid-cycle. This is around day 14, if you have a 28-day cycle.

Some women naturally have a shorter cycle of around 21 days. If this describes you, it's actually possible — though not likely — that you can conceive during or right after your period.

If you sporadically ovulate early or late, it's also possible to get pregnant by having sex right before, during, or after menstruation — but again, it's not probable.

The moral of the story? Always use birth control if you’re trying to avoid pregnancy, even if you have your period. And, if you’re trying to conceive, have sex often, but know when you’re at your most fertile. Knowledge is power!

Here’s how to figure it all out.

Timing in life is pretty much everything, especially when it comes to getting (or not getting!) pregnant. You have a fertile window of around six days each month when you're most likely to conceive. This includes:

  • the five days leading up to ovulation
  • the day of ovulation itself

Once it’s released, an egg can be fertilized for up to 24 hours.

Sounds simple enough, right? But in case you didn’t get the memo during sex ed — and lots of us didn’t, because we were too distracted by what our adolescent selves considered the "good stuff" — ovulation can be tricky.

While you’re menstruating, your body is shedding your uterine lining, because a pregnancy didn't take place last cycle. The hormones needed to sustain pregnancy, like progesterone, are very low at this time. Even so, your body is already gearing up for your next fertile window.

You may have a menstrual cycle that runs like a well-oiled machine, and then suddenly one month, ovulate a few days earlier or later than usual. You may even skip a month.

There are tons of reasons for this. For one, until we figure out how to stop time, your age is changing. Your weight may change, too, causing hormonal fluctuations to occur. Not getting enough zzz’s, or even high levels of stress, may also affect ovulation. Some women have medical conditions, like PCOS, which make ovulation super hard to predict.

Many women typically ovulate around 12 to 14 days after the first day of their last period, but some have a naturally short cycle. They may ovulate as soon as six days or so after the first day of their last period.

And then, of course, there’s sperm. It turns out those little swimmers can be pretty tricky, too.

After ejaculation, sperm may survive inside your body for up to five whole days, and can fertilize an egg at any time during that window. So even if you weren’t that close to ovulating when you had sexy time, pregnancy can still happen.

As any woman with a calendar and a bunch of best friends will tell you, the amount of days each woman spends menstruating can vary a lot.

Your menstrual flow may start to diminish and lighten in color, or turn brown towards the end of your cycle. It feels and looks like you’re still menstruating, but your body is already gearing up for your next fertile time.

If you have sex towards the end of your period, you may actually be getting close to your fertile window, especially if you have a short cycle. Let's take a look at the math.

Say you ovulate early, about six days after your period starts. You have sex on the third day of your period. The sperm have no egg to fertilize, but they're also in no hurry to die — so they hang out, doin' what sperm do.

A few days later, while they're still swimming around, you ovulate and they're drawn to that egg like a fish to water. One gets through, and there you have it — fertilization has occurred as a result of period sex.

Many women look forward to having contraception-free sex right after their period ends. It’s true that it’s unlikely you’ll get pregnant a day or two after menstruation stops, but given the lifespan of sperm and the challenges around predicting ovulation exactly — it’s not at all impossible.

This is especially true if you ovulate earlier than you usually do, or if you have a naturally short menstrual cycle of around 21 days.

Keeping in mind that your body is constantly changing, it’s pretty much impossible to ever be 100 percent safe when it comes to avoiding pregnancy, if you’re having unprotected sex.

Your menstrual cycle starts on the first day of your period, and ends on the last day before your next period starts. If you have a clockwork menstrual cycle of 28 days, you are at your “safest” — but not totally in the clear —around one week or so after you ovulate. Keep in mind that sperm can continue to live in your body, so if you’ve had unprotected sex, this sort-of-safe window may change.

If your periods are even the slightest bit irregular, so is your fertile window. And keep in mind that your cycle can change at any time, without giving you a heads up in advance.

If you’re trying to get pregnant, pinpointing ovulation is a vital first step. If you've been dutifully baby dancing mid-cycle and haven't yet gotten pregnant, you may even wonder if you have more irregular ovulation and would benefit from sex during or right after your period.

There are several ways you can try to figure out your ovulation patterns. They include:

At-home ovulation predictor kits. These tests work by detecting LH (luteinizing hormone), which surges 1–2 days before ovulation takes place. So these kits can tell you when you're going to ovulate, but they can't tell you when ovulation has taken place.

Progesterone test kits. Some women who have irregular periods, such as those with PCOS, find that using a kit that detects progesterone — the hormone released right after ovulation — is helpful to use in addition to a standard ovulation kit. Determining whether or not your body produced progesterone will help you to know if you ovulated or not.

Fertility apps. Ovulation-tracking apps compile a monthly record of multiple factors, such as basal body temperature and cervical mucus. They can help women with regular periods determine when they're ovulating. We wish we could put this in neon flashing lights, though: These apps can help you get pregnant, but they're not birth control and shouldn't be used to prevent pregnancy.

Tracking basal body temperature (BBT). Using this method as "birth control" has resulted in the birth of many babies. But, when you're trying to get pregnant, it may be effective in cluing you in to approximately when you ovulate each month.

To track your BBT, you’ll need a BBT thermometer, designed for this purpose. Take your temperature each morning when you wake up, before you move even an inch. Chart your temperature the same time of day, every day. When you chart a temperature rise of around 0.4°F for three days straight, you probably ovulated.

Remember:

Ovulation is only one factor needed for pregnancy to occur. If you've been unable to conceive after one year of unprotected sex and you're under 35 years old, see a fertility specialist. The same goes if you're over 35 and have been trying for four to six months.

If you had unprotected sex during or right after your period and wonder if you're pregnant, the short answer is — you could be. Definitely talk to your doctor or take a home pregnancy test.

You can get pregnant at any time during your cycle. Ovulation timing varies, and sperm are stubborn when it comes to their will to live. For some women that’s good news and for others, not so much.

The answer? Take control. Knowing your body, tracking ovulation, and, if necessary, taking precautions is the best way to get the outcome you want best.