How To Tell When You’re Most Fertile

Whether you're trying to get pregnant or not, getting familiar with your fertile window is a good idea. It can clue you in to when you're more likely to conceive, so you can either make a point of having sex then (if you want a baby) or be extra careful with birth control (if you don't).

Your menstrual cycle begins on the first day of a new period, Brian Levine, MD, practice director at the fertility clinic, CCRM Fertility New York. Most people's are between 28 and 32 days long, though that can vary. Ovulation — that's when the body's reproductive system releases an egg — typically occurs between days 11 and 21, and it lasts one to two days, Dr. Levine explains. That's when the body is most fertile.

But, again, these numbers are just averages. In reality, cycles vary widely — person to person, and for some people, month to month, Dr. Levine says. Hormonal birth control and health issues including endometriosis or thyroid conditions can affect cycle length and regularity. 

There are menstrual cycle tracking apps that can help you get an idea of when your fertile window might be occurring. Some apps use the “counting method." You manually input your period days, and the app flags your possible ovulation days based on your average cycle length. Others require you to take your basal body temperature; it's thought to rise for the few days you ovulate.

"In general, I would say the apps can be pretty good, but remember that they’re just guiding you,” Dr. Levine says. If the app says you're fertile for a couple days mid-cycle, assume you may be ovulating for a few days before and after too, to allow for some variation, he suggests.

But Nataki Douglas, MD, PhD, Chair of the Modern Fertility Advisory Board, says that both of these methods are less than 30% accurate. “Basal body temperature rises after you have ovulated, so this is not helpful to measure if you are trying to predict ovulation,” she notes.

To more accurately target your fertile window, she suggests using ovulation predictor kits that detect luteinizing hormones (LH), an indicator of fertility. “LH is the hormone that peaks just before ovulation, about 24 to 28 hours before an egg is released from the ovary,” Dr. Douglas says. OPKs are inexpensive, fairly accurate, and as simple to use as peeing on a stick. 

But, “These kits are most reliable for women with regular, monthly menstrual cycles,” she notes. If you have irregular cycles, you may want to see a doctor to figure out why, since this can be an indicator of an underlying health issue. "The doctor can measure estradiol, [a form of the hormone estrogen], and LH levels in your blood" to determine when you're ovulating, Dr. Douglas explains. They also might do a trans-vaginal ultrasounds to track when the ovary that is going to release the egg, she says.

Paying attention to your body can also give you a clue about when you're ovulating. For example, your cervical discharge might look like white, light mucus, Dr. Levine says. Other giveaways include breast tenderness and maybe even a heightened sense of smell. “Increased sexual desire is a sign, too,” Dr. Douglas says. This happens because of peaking levels of estradiol. These symptoms may not be as reliable as a tracker or urine test, though.

For most people, the best way to pinpoint your fertile window is likely a combination of tracking your menstrual cycles, using ovulation predictor kits, and keeping tabs on physical changes, Dr. Douglas says.

Of course, if you want to avoid getting pregnant and are sexually active, the smartest practice is to use contraceptive 100% of the time. And if you'd like to conceive, hopefully you'll be swapping your fertility app for a pregnancy app in no time.