Can You Get Pregnant on Your Period?

In short, yes. It is important to know that getting pregnant is dependent on your ovulation and is not directly linked to your period. Still, the truth is that getting pregnant during your period remains unlikely. But just how unlikely? Let us explore together!

A brief overview of periods and fertile days

The menstrual cycle refers to the recurring changes that the female reproductive system goes through. Your menstrual cycle begins on the first day of one period and ends with the start of the following period. Typically, the period comes more or less every 28 days and ovulation occurs around day 14 [1]. In theory, you can get pregnant at any time during your cycle, but you are most likely to get pregnant if you have sex on your ovulation day or roughly five days prior to it. These days are known as your fertile window [2].

However, humans are not textbooks; bodies naturally differ and so does the length of the menstrual cycle. Ovulation and — consequently — the timing of the fertile days vary from woman to woman. According to a study that took a closer look at this — periods come every 28 days on average, but for 95% of women, this happens as early as every 22 days or as late as every 36 days, or somewhere in between [3]. Moreover, that same study found that only one in four women have their fertile window between days 10 and 17. In most women, the fertile window occurs anywhere between days 4 and 23. This all relates to normal — albeit generous — variability observed in women with regular cycles. Naturally, greater variability is observed in women who get their periods irregularly.

Let’s dig deeper

Now we know that your fertile window can occur sooner or later, making your chances of getting pregnant on your period higher or lower. Nonetheless, that’s not the complete tale. Even though the fertile window by definition closes on the day you ovulate — and includes about five preceding days — for a long time little was known about when it emerges. The first study that sought to define the starting moment of the fertile window also disproved the traditional view that the fertile window is fixed to six days [4]. Rather, the fertile window length varies considerably; some women have a single fertile day in their cycle while others have more than five. Not only does the fertile window differ between women, but it can also differ within any one woman — you can have different fertile window lengths between your cycles [5]. Still, while the fertile window is certainly not fixed to six days, it is also unlikely to exceed that number. Recent research — published in the same journal as the one that initially disproved the paradigm — suggested that the number of days of a distinctly fertile cervical mucus, when you are at the highest chance of getting pregnant, corresponds to the prior view of a fertile window lasting six days in total [6]. In any case, the length of your fertile window is tightly linked to your chances of getting pregnant in general and during your period.

Let’s dig even deeper

Even if unpredictability around the menstrual cycle did not exist — which it does — and if chances of getting pregnant on your period were zero — which they aren’t — bleeding would still not be a good enough indicator of your infertile days. Why? Well, one reason is that uterine bleeding other than period can occur as mid-cycle spotting that is narrowly linked to your ovulation [7]. Mistaking such bleeding for a period would mean having sex during the kind of bleeding that dramatically increases your chances of getting pregnant. Another reason is that a woman can bleed while she is already pregnant. Can you have a period during your pregnancy, you may now wonder? Not really. It is just that cyclic period-like bleeding or other kinds of bleeding while pregnant — particularly early on — are not uncommon [8, 9, 10]. Usually short and light, bleeding during early pregnancy appears in nearly one of ten pregnant women. Clearly, confusing any type of pregnancy-related bleeding or period-like cramps during pregnancy for some of the classic symptoms of a period, makes debating the chances of getting pregnant on your period utterly paradoxical.

How likely is it to get pregnant during your period?

The false belief that a woman can’t get pregnant during her period is rooted in the presumption that the period is separate from the fertility window in the menstrual cycle. Yet, this is not always the case.

We already discussed differences in cycle length between women. However, there are also notable differences in cycle lengths of any one woman — less than one in a hundred women have the same cycle length across four consecutive cycles. A shorter cycle opens up the possibility of sooner ovulation. Sooner ovulation opens the possibility of your most fertile days overlapping with your period. For example, research suggests that the probability of ovulating on day 10 is generally 0-2% for cycles of length between 25 and 35 days [11]. But this probability jumps to 5 and 9% for cycles of length 24 and 23 days, respectively. That is, nine in a hundred women with a short cycle of 23 days will ovulate on day 10 and potentially have their fertile window open on day 5. There is a far lower possibility of ovulating any time before day 10; for cycle length between 23 and 35 days, research suggests that only the shortest cycle length of 23 days carries some — rather low, 1% — chance of ovulating on day 7, in which case your most fertile days may begin on the second day of your period.

Let’s now assume that you have a short cycle and that you ovulate on day 7. Could you have sex on the third day of your period and get pregnant? To answer this, we need to examine the lifespan of sperm cells.

How long can sperm survive after sex?

There is nothing arbitrary about the arrival of an adequate sperm cell to the fallopian tube where it unites with an egg cell. Fertilization is a pinnacle of well-concerted interactions between sperm cells and a woman's reproductive tract. Before its arrival to the fallopian tube, the sperm cell must complete a strenuous journey, conquering anatomical barriers and undergoing physiological changes. The length of this journey depends on the time between sex and ovulation as well as on how long sperm cells can survive in a woman’s body [12]. If you were to have sex on your ovulation day, that journey could be as brief as ten minutes. But what if you were to have sex several days before you ovulate? Could sperm survive that long? Actually, yes. The main supporter of sperm’s lifespan is fertile cervical mucus. Your cervix secretes different types of discharge throughout your cycle. As ovulation nears and your estrogen levels rise, your discharge becomes wet, stretchy, and slippery. This estrogenic mucus, which performs several fertilization-supportive functions, might have derived from an evolutionary need for filtering out abnormal sperm — which can amount to 95% of the total sperm in one ejaculation! It is this type of friendly cervical environment that nurtures normal sperm cells, helping them survive for up to six days.

Pregnancy at the beginning vs. end of a period

The average period length is five to six days. Less than 5% of women are on their period for less than four days, and merely 5% of women have periods that last eight days or longer [13, 14]. As previously mentioned, ovulating on day 10 is fairly rare and chances to ovulate any time before are negligible. However, in the rare event of ovulating on day 10, your fertility window — if long enough — could begin at the very end of your period. But what if you have particularly short or long periods? Does that mean that your short period cannot possibly coincide with the fertility window, or that your long period largely overlaps with your fertile days? Likely not. As research suggests, very short cycles come with shorter periods, whereas long cycles come with slightly longer periods [15]. That being said, if your period is only three days long, you likely have a shorter cycle that is associated with an earlier fertility window and you could still end up getting pregnant at the end of your period. And if your period is more than eight days long, your menstrual cycle is probably a longer one that comes with a later fertility window, too. Nonetheless, your period could last long enough for you to get pregnant towards its end. Nevertheless, while the likelihood of getting pregnant increases each day of your period, it is nearly zero at its very beginning [2].

When to have sex if you want to get pregnant?

If you want to get pregnant, you should have sex shortly before — not after — you ovulate. This may seem counterintuitive because the egg is still not there. Even so, we learned that sperm cells can survive a few days in the female reproductive tract; they do not need to meet with your egg cell the very moment it is released. Besides, an egg cell dies quickly, making sex two days before ovulation the best timing if you seek to get pregnant [16]. Despite that, there is one better approach than timing your sex — tracking your estrogenic discharge. Research suggests that the highest pregnancy probability for having sex two days before ovulation when there is no discharge is still lower than the pregnancy probability on any day in the six-day interval when estrogenic discharge is present [17].

Why does tracking your menstrual cycle matter?

Recognizing estrogenic mucus can be a complicated and frustrating experience for some women. Still — regardless of whether you seek to get pregnant or not — knowing your menstrual cycle is fundamental to sound reproductive health. It really is an act of self-care. An alternative to observing discharge are the apps to track fertility or hormonal tests you can do in a laboratory. But the majority of the apps predict ovulation based on cycle length only or falsely assume that each woman’s fertile window is of the same length. Laboratory tests, on the other hand, require more effort than most of us are willing to give regularly.

So, what is an easier approach, then? Is there something as convenient as surfing an app from your couch, but at the same time as accurate as laboratory tests? Well, meet inne. With inne, you can collect a sample of your saliva and learn your progesterone levels within minutes. Progesterone is a telling hormone because its levels grow right after your ovulation. While science has observed its sharp post-ovulation rise in the past century, it wasn’t until inne that you could detect progesterone levels from the coziness of your own home [18, 19]. Moreover, the real-time hormonal fluctuations that inne stores in your smartphone are valuable insights that can help your doctor make more informed decisions about your body. Importantly, inne does not make assumptions based on averages that do not always apply. Rather, inne reads your body and teaches you more about your unique self.

References

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