When are you most fertile? How to tell when you're more likely to get pregnant

Whether you’re trying to get pregnant or trying to make sure you don’t, understanding a few fertility basics is key.
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By Caroline Moss

If you’re anything like me, you realized pretty late in life that all those teen shows and movies that taught us pregnancy was an inevitable and absolute consequence of sex were, well, kind of lying to us.

Yes, of course, you can get pregnant from sex. And when you’re not trying to get pregnant, you should be using precautions to prevent pregnancy whenever you’re sexually active.

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But, unlike what was drilled into our minds growing up to scare the heck out of us, you aren't able to get pregnant every single time you have sex. This tiny piece of sex education — the fact that there are specific days when you can and can’t get pregnant — was as much a mystery to me as Santa’s real identity until I was well into college.

The truth is that most women only have a short window each month that they will be able to get pregnant. Sometimes, the window is just hours long.

Whether you’re trying to get pregnant or trying to make sure you don’t, understanding a few fertility basics is key. Here’s how to figure out when you’re most fertile.

Take a closer look at your cycle

Fertility is linked to menstruation. If you get your period like clockwork once a month and can sense it coming from a mile away, it’s likely that you’re on what medical professionals call a “regular” 28-day cycle. If this is the case, you start ovulating on day 14 of your cycle (the cycle starts on the first day of your period). Dr. Richard Paulson of USC Fertility in Los Angeles, California, says that this can lead to some confusion. “You want to track 14 days before your next period,” he explained. “Not 14 days after your last one.”

Of course, if you have a 28-day cycle, those days end up being the same. But many people do not. Fourteen days before your next period is when your fallopian tubes release an egg to go meet up with some sperm and maybe start creating an embryo. It’s usually a one-day occurrence according to Paulson, but he recommends starting to have sex with your partner for a few days before you hit that 14-day mark on the calendar if you're hoping to conceive. “If you miss it, you miss it,” he says.

“A big misconception is that you should have sex five times on that one day,” Paulson told TMRW, but the number of times in a day won’t matter. Starting prior to the “big day” can help get the sperm inside to be ready to meet the egg when it drops. According to the American College of Obstetricians and Gynecologists (ACOG), an egg can only be fertilized for about 24 hours following ovulation, while a man's sperm can live in a woman's body for anywhere between three and five days.

When are you most fertile if your cycle is irregular?

Ovulation and menstruation apps can help track your cycle and enable to better predict when you're in the window of fertility. myflo period tracker

It's a little more difficult to predict, but according to Paulson, women typically ovulate 14 days before the start of their next period. So if your cycle is 35 days, you're most likely ovulating on the 21st day since the start of your last period.

Lots of menstruation apps offer calendars and reminders for easy pregnancy planning, which can be particularly helpful since different months have different numbers of days and it can be hard to keep track. Visualizing your cycle with a calendar app will help you determine your window, especially if your period is less reliable from a timing perspective. (Paulson says the apps work very well, but that every patient he sees comes in using a different one, so he has no specific recommendation.)

Other methods of tracking ovulation if your flow is inconsistent can include monitoring your cervical mucus and basal body temperature. According to ACOG, the amount of discharge you experience increases just before ovulation and becomes thin and slippery. Recognizing those changes can be an indicator that you're within the fertile window.

Your basal body temperature your temperature when you're at rest, and according to ACOG, it rises slightly during ovulation (we're talking less than half a degree to 1 degree Fahrenheit). If you're looking to plan a pregnancy, it may not be the most effective way to conceive, simply because the temperature only changes once you've already ovulated, but paired with other tracking methods it can be a useful piece of insight into what's happening in your body.

That’s the basics of fertility but like I said, everyone’s body is different.

What other ways are there to track overall fertility?

If you want an even clearer picture of what's happening inside your body, you can ask your doctor for blood work to determine overall fertility. This will provide insight into how many eggs you have by measuring the level of anti-mullerian hormone in your blood and measure other relevant hormone levels that can indicate if you have conditions like polycystic ovarian syndrome or thyroid disorders that could impact your ability to get pregnant while trying to conceive.

A comprehensive hormone panel can provide more detailed information about the specifics of your individual fertility level. Getty Images

Because those tests can sometimes be expensive, start-ups like Modern Fertility also offer at-home tests and give you similar insights that you can then bring to your primary care doctor or OB-GYN to talk about next steps.

“Having all that information available for you as you begin to plan for your family’s future will end up giving you more options,” Paulson.

The most important thing about fertility is that there is no one size fits all to pregnancy, but knowing the basics is the exact right place to start.