When Are You Most Fertile?
If you’re trying to get pregnant, you might be wondering: When are you most fertile? You might have heard about something called the fertile window. But what does this “window” really refer to—and how important is it to pinpoint? The short answer: Your fertile window is the name for the six-day range each month that you can conceive. Pinpointing it accurately is the key to getting pregnant, as these are the days that a healthy sperm can fertilize a viable egg.
Here, we will review: how to pinpoint your fertile days and why it’s essential to do so.
When are you most fertile?
Like many things in life, when it comes to getting pregnant—timing is everything. During the average woman’s menstrual cycle, there are six days when sex can result in a pregnancy. These days, the five days before ovulation and the day of ovulation itself, are your fertile days.
The fertile window extends for six days because sperm can live for up to five days in the female reproductive tract. So, if you have sex in those five days pre-ovulation, the sperm can wait it out in your fallopian tubes for a dominant follicle to rupture and release an egg. Or, if you have sex on ovulation day, the sperm and egg can arrive in the fallopian tubes around the same time. (Though research has shown that when conception occurs on ovulation day itself, it is more likely to result in a what’s called a chemical pregnancy or a loss.)
Here are your most fertile days, ranked in order:
|2-3 days before ovulation||20 – 30 percent chance of conception|
|4 days before ovulation
||10 – 12 percent chance of conception|
||10 – 12 percent chance of conception|
|5-6 days before ovulation
||0 – 5 percent chance of conception|
The most important thing to note here is that you can conceive anytime during the five days before ovulation and on ovulation day itself. Within your six-day fertile window, the two to three days before ovulation are your most fertile.
The fertile days are around cycle day 14 right? (Not necessarily.)
The reality is: menstrual cycle lengths vary. The basic understanding of the menstrual cycle was developed between 1910 and 1960. And at that time, research established that the average cycle was 28-days long and ovulation tended to occur on day 14. And following those established norms, little research was conducted on the variability of women’s menstrual cycles.
But more recent research, like this study—an extensive menstrual cycle study which examined 184 women attempting to conceive—uncovered the variabilities that exist in the female menstrual cycle. This study looked at 58 nonconceptive and 111 pregnancy menstrual cycles. The results of this study were consistent with earlier research on some points, such as the conclusion that the average length of woman’s cycle was about 28 days. But what this study uncovered was that there was variation among when implantation occurred, as well as variation in when ovulation happened.
So, I just need to know when I’m ovulating, right?
Well, what you really want to narrow in on is the 2-3 days before ovulation. And what makes that a challenge is that ovulation can vary from woman to woman and even from cycle to cycle. One study found that women ovulated anywhere from cycle day 8 cycle day 26. (And this was just one small study in North Carolina that examined 221 healthy women for one menstrual cycle.)
Why does this matter? It’s important to point out because women have long been told that ovulation happens on or around day 14. And if you look at ovulation dates across a large group of women on average, it does. But lumping all women into one bucket and spitting out an average number doesn’t tell you anything about when you, or anyone else individually, will ovulate.
Only a small percentage of women ovulate on day 14
According to several studies, like this one, only a small percentage of women ovulate exactly on day 14. This is true even for women who have regular cycles. For women with irregular cycles, the fertile window is even trickier to pinpoint. So, while the day 14 average is useful for estimating when women—as a population—tend to ovulate, it’s not very helpful for pinpointing when a woman—as an individual—ovulates.
Most women’s fertile window falls outside the clinical guidelines (that is, between cycle days 10 and 17)
Again, for years, women have been told that ovulation happens around cycle day 14. But the North Carolina study found a wide range in ovulation dates and discovered that most women’s fertile windows (over 70%) didn’t fall entirely within that range. This chart below shows the full distribution of the fertile windows recorded among the women in the study.
What does this mean? Even if you feel that you have a “normal” menstrual cycle, it is quite possible—perhaps even likely, that your fertile window will not fall entirely within the clinical guidelines.
If you want to get pregnant, you need to know your fertile window
If you have a regular cycle, this is easy—right? Wrong. That same study found that the fertile window occurred during a broad range of days in the menstrual cycle. OK, well, if a lot of women fall outside of that 10 to 17-day range, you can just extend it, right? Let’s say you broaden that range to having sex between days 6 and 21. You’re likely to be in your fertile window during those days, right?
Again—wrong. The same study found that on every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window, and pointed out that using the clinical guidelines to estimate ovulation is outdated.
The takeaway? If you’re just guessing at your fertile window, it’s possible you’ll miss it.
So, when do I ovulate?
Most important takeaway: ovulation follows a series of hormonal events that happen each month, but the pace at which it unfolds can vary. Pinpointing ovulation (and your fertile days) is a bit of a guessing game, but there are signs you can look for, and tracking your cycle can help.
Where can I find out more about ovulation?
Here at Ava, we’ve become experts in the science of and research about ovulation, and how to pinpoint the fertile window. You’ll find fact-checked posts that explain:
- what ovulation is
- when ovulation occurs
- why to look at some ovulation trackers with a skeptical eye
- overviews on ovulation symptoms and ovulation pain
Even if you have a regular cycle, this doesn’t mean you have regular ovulation
When women have “regular” cycles, which are about 28-days or a month apart, the assumption is that they are ovulating regularly. (And many times, they are!) But Dr. Jerilynn Prior, Professor of Endocrinology and Metabolism at the University of British Columbia, points out in her research that ovulation is highly variable for all women.
Dr. Prior authored an extensive study of over 3,100 healthy, regularly menstruating women between the ages of 20 and 49.9, which found that only 63% of these women ovulated during the cycle that was tested, meaning that 37% of them did not ovulate and had what’s known as an “anovulatory” cycle. She and the other study authors concluded that anovulation (in a random population) occurs in over a third of clinically normal cycles.
The takeaway? A “normal” length cycle doesn’t necessarily mean that it was an ovulatory cycle. Sometimes, even healthy, normally menstruating women can have a sporadic nonovulatory cycle, meaning that ovulation didn’t occur that month for any variety of reasons—stress, illness, or travel to name a few possibilities. And in most cases, it’s nothing to worry about.
The key to getting pregnant is to zoom in on your fertile days
The good news is, there are a number of reliable ways to narrow in on your fertile window. Here are some options for tracking fertile days:
A few days prior to ovulation, your cervical mucus will begin to become slippery, resembling egg whites. This is a symptom of ovulation and a sign that you are fertile. When you see discharge looking like this, take it as a sign that you’re in your fertile window.
An ovulation test (also called an OPK for “ovulation predictor kit”) detects the concentration and presence of luteinizing hormone (LH) in your urine. Around 12-36 hours prior to ovulation, there’s a surge in LH levels. This surge can last anywhere from a couple of hours to a couple of days. The length of the surge isn’t important in determining when ovulation happens, as most women tend to ovulate about 24 hours after the surge begins. Unfortunately, LH tests can’t tell you if your surge just began or if it’s nearing the end. The problem for TTC? By the time you get a positive LH test, there may not be much (or any) time left before ovulation—and you may have missed some of your most fertile days already, depending on the timing of your LH surge.
Basal body temperature
Tracking your basal body temperature (or BBT) is another option. With this method, you take your temperature at the same time each morning—before you get out of bed or move around at all. You also need to use a special thermometer specifically designed to measure basal body temperature. The problem with the BBT method is that your basal body temperature rises after ovulation. So once you detect that temperature increase, your fertile window for that cycle has already closed.
This wearable device is clinically proven to detect the very beginning of the fertile window—in real time, as it happens. You wear the bracelet each night while you sleep. Ava’s proprietary algorithm uses machine learning and artificial intelligence, combining knowledge of your cycle with the physiological data collected (parameters like resting pulse rate, breathing rate and skin temperature, to name a few) to predict your fertile window accurately.
Here are some additional resources that the team at Ava has developed:
- Our fertility calculator will take you through a brief quiz (gathering data about your lifestyle, health, cycle, and partner), and will give you a ballpark figure for how long it will take you to get pregnant.
- Here, we’ve outlined a fertility calendar, that will help you identify the best days to have sex to get pregnant, backed by research.
- An overview of how tracking your cycle can help you pinpoint your fertile days.
- An explanation of how the Ava algorithm works.