The simple answer to wether not not you can get pregnant on your period is that while it is less likely, it is still possible. The reason for this is that sperm can survive for up to five days inside the female reproductive tract, meaning that if a woman ovulates shortly during or after her period ends, there is a chance that sperm from intercourse during her period can fertilise an egg. Ovulation can not be predicted.
The likelihood of becoming pregnant during a period is significantly lower than at other times in the menstrual cycle. This is because the hormone levels are at their lowest point during that time. Additionally, the shedding of the uterine lining during menstruation makes it less hospitable to a fertilised egg. The likelihood of becoming pregnant during a period can increase in certain circumstances, such as during perimenopause or in women with early ovulation. Perimenopause is the transitional phase that precedes menopause, during which a woman’s hormonal balance changes, and her periods become irregular. During this phase, estrogen levels can fluctuate greatly, leading to very high estrogen levels and irregular ovulation patterns. This can increase the chance of becoming pregnant during a period since ovulation may occur earlier than usual. While this is commonly seen around ovulation, any women with early ovulation may also have a higher chance of getting pregnant during their period. This is because they may ovulate soon after their period ends, increasing the likelihood of sperm meeting an egg. However, early ovulation can also lead to fertility issues and increased risk of miscarriage, as the egg is not allowed adequate time to fully develop and mature properly with early ovulation. The window of opportunity for conception is also smaller than in women who ovulate later in their cycle.
The difference between a true period and inter-menstrual bleeding
Many women falsely assume that they have gotten pregnant while on their period but in reality may not have been able to identify their true period. Stories like this lead women to distrust FAM as a reliable method of birth control and perpetuates the myth that “you can get pregnant any day of your cycle” which is not true. To better understand the relationship between periods and fertility, it is important to understand the difference between a true period and inter-menstrual bleeding. A true period is the shedding of the endometrial lining that occurs when an egg released during ovulation is not fertilised. A true period must always follow ovulation. On the other hand, inter-menstrual bleeding, or “abnormal uterine bleeding” is bleeding that occurs outside of a woman’s normal menstrual period. This can be caused by a variety of factors, including hormonal imbalances, stress, pregnancy, or certain medical conditions. Inter-menstrual bleeding can range from some light spotting to heavy flow that goes on for days. Most women will experience some inter-menstrual bleeding at some point in their lives. In some cases, it may be considered normal but in other cases it can signify a deeper hormonal problem such as PCOS.
Personally I have very regular cycles but have experienced inter-menstrual bleeding around 5-6 times in my life. The first was when coming off birth control, when I was not ovulating. I experienced light brown flow that would last 12 days around 6-7 weeks apart. This was caused by lack of ovulation and my body attempting to establish a regular cycle. This pattern continued for a year until my body ovulated again. The next was two seperate instances where I had anovulatory bleeds due to high situational stress. The stress had caused me to skip ovulation and instead of getting my regular period every month, I had a bleed around 17 days later than I was supposed to. The bleed was bright red, somewhat lighter than usual but still moderately heavy and not painful. It could easily have been mistaken for a period, but it wasn’t because I had not confirmed ovulation. It also did not feel like a real period as my body did not go through its usual hormonal changes. The last few times I had inter-menstrual bleeding was light spotting around the time of ovulation. In this case, if I was not charting my cycle, I may have assumed my period was starting, but in fact this was a highly fertile time. The bleed was caused by the hormonal changes at ovulation itself. Some spotting around ovulation is considered normal, but it is also common for some women to have heavier bleeds with their ovulation especially when hormones are calibrating such as puberty, peri- menopause, post-partum and coming off hormonal birth control. This is why charting the menstrual cycle is so important. Many women can detect temperature shifts to confirm ovulation during a bleed.
In fertility awareness methods, any bleeding is considered as potentially fertile, and in fact, inter-menstrual bleeding often occurs with ovulation itself and is considered a highly fertile time. To identify wether a bleed is a true period or inter-menstrual bleeding, it is important to learn to chart the menstrual cycle and learn how to identify ovulation. This also encourages body literacy and may help to identify hormonal imbalances when there is by assessing causes for inter-menstrual bleeding. When using FAM, all bleeding should be charted and treated as potentially fertile.
Davis E, Sparzak PB. Abnormal Uterine Bleeding. [Updated 2022 Sep 9]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532913/