When Are You Least Likely to Get Pregnant? Understanding Low Fertility Windows

Many people track their cycles to better understand their bodies, whether to try to conceive, avoid pregnancy, or just get in tune with what’s happening each month.

Here’s the truth: while there are certain times in your menstrual cycle when you’re less likely to get pregnant, there’s no day that’s 100% “safe” from pregnancy if you’re having unprotected, penetrative sex. But some days are definitely lower risk than others. Those “less fertile” windows tend to happen when your body is far away from ovulation (the time when an egg is released).

You’re least likely to get pregnant in the days outside your fertile window, but “least likely” doesn’t mean “impossible.” In this guide, we’ll unpack when your risk of pregnancy is lowest, why it’s never zero, and how factors like stress, hormones, and irregular periods can shift things around.

The concept of the fertile window and the fertility probability curve

Let’s start with what we mean by the fertile window. Every menstrual cycle, your body goes through a rhythm of hormone changes that build up to ovulation. Once that egg is released, it only lives for about 12 to 24 hours. But sperm? They can survive in your reproductive tract for up to five days.

Because of that, your fertile window is usually defined as the six days leading up to and including ovulation. That’s the stretch when sex is most likely to lead to pregnancy. If sperm are already hanging around when the egg drops, conception can happen pretty easily.

If you picture this as a graph, it looks like a hill: fertility is lowest right after your period, starts to rise a few days before ovulation occurs, peaks on the day before and the day of ovulation, and then drops sharply afterward.

Your chances of getting pregnant are highest if you have unprotected sex a day or two before ovulation. But three or four days before ovulation, it might only be a few percent. And outside that six-day fertile window? The probability gets much lower, but it never disappears entirely.

Your hormones, cycle length, and even daily stress levels can nudge ovulation earlier or later than expected, which is why no timing method is foolproof.

When is a woman least fertile — cycle days with the lowest chance

So, when are you least likely to get pregnant during your cycle? In a typical 28-day cycle (though this is just an average, and most people's cycles are longer or shorter), the chances of pregnancy are lowest at the very beginning and very end of the cycle.

During menstruation, which usually lasts from days one to five, the uterus sheds its lining, and hormone levels are at their lowest. Since there’s no mature egg available during this time, and the body is just starting to prepare for the next menstrual cycle, these days are among the least fertile.

Following menstruation is the early follicular phase, which happens around days six to nine. During this time, you’re still in the early part of your cycle, and ovulation hasn’t occurred yet. Although the risk of pregnancy remains low, it may begin to increase, especially for those who ovulate earlier. Research shows that some individuals can enter their fertile window as early as day six, depending on the length of their menstrual cycle.

After ovulation comes the luteal phase, which lasts roughly from days 20 to 28. Once the egg has died, there’s nothing left to fertilize. During this phase, the body either prepares for a potential pregnancy or gets ready for the next period, resulting in very low fertility once again.

So, if you had to name your lowest-risk zones, they’d be:

• During your menstrual period (especially the first few days)
• The few days right after your period ends
• The last few days before your next period starts.

But keep this in mind: your menstrual cycle isn’t always the same month to month. You might ovulate a few days earlier or later than you expect, which means even those “safe” days can shift around.

Factors that shift “least fertile” days and make them unreliable

Here’s where things get tricky. Your menstrual cycle might look pretty regular on paper, but the timing of ovulation can move around, and that makes predicting your least fertile days kind of like trying to hit a moving target. A few of the most common reasons your cycle might change:

• Stress and lifestyle shifts: Big life changes, travel, sleep disruptions, or even workouts can delay or speed up ovulation. Stress hormones can easily throw off your reproductive hormones.
• Illness or medications: Being sick, taking antibiotics, or starting/stopping hormonal treatments can all tweak your menstrual cycle.
• Hormonal imbalances: Conditions like polycystic ovary syndrome (PCOS), thyroid issues, or perimenopause can make ovulation unpredictable — or sometimes skip it altogether.
• Age: As we get older, fertility starts to decline, which can make cycles shorter and more irregular. That means ovulation can shift or even double up in a single cycle.
• Misread signals: Spotting, breakthrough bleeding, or light cycles can make it hard to tell what your true “day one” is. That throws off the count for everything else.

Even if your menstrual cycles are normally like clockwork, small variations happen all the time. One study found that people with “regular” 28-day cycles still ovulated anywhere between day 11 and day 20. So while it’s helpful to know when you’re usually least fertile, remember that biology doesn’t always follow the calendar.

Methods people use to estimate low fertility — and their limitations

If you’re curious about using your cycle to understand when you’re least likely to get pregnant, there are several fertility awareness methods (FAMs) you can try. These methods (sometimes called "natural family planning") involve tracking signs your body gives — like cervical mucus, basal body temperature, and cycle length — to estimate when you’re most and least fertile. These tools can help you tune in to your body’s natural rhythms, but they all come with their own quirks and learning curves. The “best” method isn’t necessarily the one that’s most scientifically reliable on paper; it’s the one that fits your lifestyle, cycle regularity, and how much daily tracking you’re comfortable with.

Here are the different fertility tracking methods:

"Rhythm" or calendar method

This is the classic approach, and involves tracking your period dates for several months to estimate ovulation. For example, if you have cycles that last between 26 and 32 days, you might assume your fertile window is between days 8 and 19, leaving the others “lower risk.” It’s simple, but only works if your cycles are super consistent.

Basal body temperature (BBT) tracking

You take your temperature every morning before getting out of bed. After ovulation, your progesterone levels rise, which slightly increases your resting body temperature. Once you notice that temperature shift, you know ovulation has already happened. The catch? It only tells you after the fact, so it helps identify when you’ve already left your fertile window, not when you’re about to enter it.

Cervical mucus method

Your cervical fluid changes texture and appearance depending on where you are in your cycle. Around ovulation, it becomes clear, slippery, and stretchy (like raw egg whites). When your mucus is thicker, cloudy, or dry, you’re more likely to be in a lower fertility phase. It’s a great natural indicator, but it takes practice to read correctly. You can track your cervical mucus by paying attention to the color and consistency of your vaginal discharge.

Symptothermal method

This one combines temperature tracking, mucus observation, and sometimes cervical position. Because it pulls from multiple signs, it’s more reliable, but also more involved. It requires daily attention and consistency.

TwoDay method

A simplified mucus-based rule: if you notice cervical mucus secretions today or yesterday, consider yourself fertile; if not, you’re probably not. It’s easier but less precise.

While FAMs can be effective when used perfectly, real life isn’t perfect. Typical use failure rates are about 23% per year, because their effectiveness depends on correct and consistent use. They work best when paired with other forms of birth control (like condoms), especially if preventing an unwanted pregnancy is a top priority.

Practical guidelines: when to consider days “safer” (though not completely safe)

If you’re going to use cycle tracking or natural methods, here are some gentle guidelines to help minimize risk:

• During your period and the days right before your next one are usually low-fertility days. That’s when your hormone levels and ovulation potential are at their lowest.
• The days right after your period ends might also be relatively low risk, but only if you have a longer cycle (say, 30 days or more). If your cycles are shorter, you might be ovulating sooner than you think.
• If you’re not using hormonal birth control, pairing awareness methods with condoms or another barrier method adds an extra layer of safety.
• And if something seems off — like you’ve been stressed or your period came late — it’s smart to assume you could be more fertile than expected.

In other words: treat “safer days” as lower risk, not no risk. Your body is dynamic, and even the best predictions can miss a surprise ovulation.

Safe methods to prevent pregnancy

If you really want reliable protection, some methods don’t depend on tracking or timing at all. Here’s a rundown of the most effective ones:

• Hormonal birth control: Pills, patches, vaginal rings, injections, and implants all work by preventing ovulation. They also thicken cervical mucus and thin the uterine lining, creating multiple barriers to pregnancy. When used correctly, they’re over 99% effective.
• Intrauterine devices (IUDs): There are two main types — hormonal (like Mirena or Kyleena) and non-hormonal (the copper IUD). Both are long-lasting and over 99% effective, and neither requires daily maintenance.
• Barrier methods: Condoms (internal or external), diaphragms, and cervical caps create a physical barrier between sperm and egg. They’re less effective than hormonal methods on their own, but they’re great for STI protection and can be combined with other methods.
• Permanent options: Tubal ligation or sterilization is an option for people who are certain they don’t want children in the future.
• Emergency contraception (EC): If you have unprotected sex or your primary method fails, EC (like Plan B) can prevent or delay ovulation if taken soon enough.

For those who prefer natural or non-hormonal methods, combining awareness with condoms or a copper IUD can offer both confidence and flexibility.

The bottom line is that it’s totally okay to be curious about your body and timing, but if you’re trying to prevent pregnancy, pairing awareness with reliable birth control is the safest way to go.

FAQs about low fertility windows

What is the least likely day to get pregnant?

You’re least likely to get pregnant at the very beginning of your period, especially the first two or three days of bleeding. At this point, your body is shedding the uterine lining that would support a pregnancy, your estrogen and progesterone levels are at their minimum, and no mature egg is present. However, “lowest” doesn’t mean “zero.” In people with very short cycles (say, 21–24 days), ovulation can happen sooner than expected, sometimes as early as day 10. Since sperm can live in the reproductive tract for up to five days, having sex toward the end of your period could, in rare cases, line up with an early ovulation and lead to pregnancy. So while the early days of your period are typically low-risk, day six (and beyond) can’t be considered completely off-limits.

When is pregnancy least likely to occur?

Pregnancy is least likely during times when your reproductive system simply isn’t set up for fertilization. This includes your menstrual phase (when your body is shedding the old uterine lining and gearing up for a new cycle) and the late luteal phase (the days right before your next period). During these stages, your hormone levels (especially estrogen and luteinizing hormone) are too low to trigger ovulation, and mucus is thicker and more acidic, which makes it harder for sperm to survive. Your body is essentially in a “reset” or “cool-down” mode between ovulations. That said, everyone’s cycle is a little different. Some people ovulate early or late, meaning that their fertile window could creep closer to (or even overlap with) what would normally be considered a “low fertility” time. So while these are the days when pregnancy is least likely, they’re never completely foolproof.

When is it most likely to get pregnant?

You’re most likely to get pregnant during your fertile window, which covers the five days before ovulation plus the day of ovulation itself. During this time, your cervical mucus becomes slippery and stretchy (perfect for helping sperm travel), and estrogen levels surge to support the release of an egg. The single most fertile days are usually the two days before ovulation and the day of ovulation. Intercourse during this window gives sperm the best chance of being in the right place at the right time. If you’re trying to conceive, tracking signs like discharge changes, a slight drop, then a spike in basal body temperature, or results from an ovulation test (LH surge) can help you zero in on this fertile phase. If you’re trying to avoid pregnancy, those same days are the ones to be extra cautious with protection or abstain from unprotected sex altogether.

What is the lowest chance I can get pregnant during the low fertility window?

The numbers vary depending on your cycle, age, and overall reproductive health, but outside the fertile window, your chances of conceiving are low. There’s always a margin of error because ovulation doesn’t follow an exact schedule. Stress, travel, illness, or just natural variation can make your ovary release an egg a few days earlier or later than usual. Since sperm can live for several days, sex that happens during what you think is a “low fertility” phase could still lead to pregnancy if ovulation surprises you. If avoiding pregnancy is your goal, it’s safest to combine awareness with reliable contraception (like condoms, a hormonal method, or an IUD).

What is the “safe period”?

The phrase “safe period” is an older way of describing the days in your menstrual cycle when you’re least likely to get pregnant. Today, many healthcare professionals prefer the term “low fertility window” because it’s more accurate and acknowledges that no day is ever completely risk-free. Generally, the “safe period” covers the days during your period (when an egg hasn’t been released yet), and the late luteal days before your next period starts (when the egg from that cycle is already gone). However, ovulation isn’t always predictable. It can shift due to stress, sleep changes, illness, or just natural variation between cycles. That means a day you thought was “safe” one month might fall closer to your fertile window the next. So instead of thinking about a “safe period,” it’s more realistic to think in terms of risk levels that rise and fall throughout your cycle — and to pair cycle awareness with a backup method if pregnancy prevention is important to you.