A scientific argument for why women can't get pregnant during their periods might go something like this:
But getting pregnant involves plenty of intricacies and uncertainties. In fact, the menstrual cycle's variability is a primary reason why having a period doesn't temporarily render women infertile.
The typical menstrual cycle lasts 28 days, split into the follicular and luteal phases. In a nutshell, the follicular phase consists of a healthy egg developing in the ovaries, while the luteal phase is the post-ovulation preparatory period for that egg to meet up with a sperm in the fallopian tubes.about.com
A more detailed daily breakdown of how all of that happens within a single cycle goes as follows:
Days 1 -- 5: Menstruation commences as the lining of the uterus called the endometrium sheds and an unfertilized egg exits the uterus.
Days 6 -- 10: Estrogen promotes the regrowth of endometrial cells in the uterus in preparation to receive an egg once again.
Days 11 -- 18: Ovulation, the peak of female fertility, happens when a mature follicle releases an egg into the fallopian tubes.
Days 19 -- 28: Hormone secretions prompt the uterine lining to continue thickening to create a nurturing environment for embryo implantation should fertilization occur.parents.com
So, can a woman who has sex during her period get pregnant or not?
FACT: While a woman is unlikely to get pregnant during her period, it is absolutely possible
A woman can become pregnant from having sexual intercourse during menstruation. In fact, recent research conducted by the United States National Institutes of Health (NIH) suggests that women have the potential to become pregnant at virtually any time during their cycle. While the chance of pregnancy is greater on certain days of a woman's cycle, it is never completely absent.wisegeek.org
If a woman is hoping to get pregnant, it's commonly assumed that her best chance falls sometime around 14 days after her last period. But that rule of thumb only applies for those with a textbook 28 day menstrual cycle.
More generally, women may reach peak fertility between 11 and 21 days after their periods, depending in part on the frequency and duration of their menstruation. Irregular periods that only come around every few months, as well as cycles shorter than the average 28 days may hold higher probabilities for overlap between ovulation and menstruation. Although the statistical chances of it happening are low, there are three main ways that it's possible to get pregnant from unprotected sex despite the presence of vaginal bleeding.howstuffworks.com
First up, if a woman has an especially short menstrual cycle, ovulation can occur before menstruation stops [source: Hirsch]. This is the least likely scenario for a period pregnancy to take place, but a longer period lasting up to seven or eight days combined with a compressed menstrual cycle can add up to an embryo.care2.com
More commonly with shorter menstrual cycles, the ovaries could release an egg a few days after menstruation finishes. But even though an egg wasn't ready and waiting during the period itself, unprotected intercourse on, say, the last day of bleeding could leave behind sperm with a surprisingly potent shelf life. Sperm can survive inside a woman's body for three to five days, possibly resulting in pregnancy if the ovaries release an egg into the fallopian tubes in that time frame.babycenter.com
Vaginal bleeding can also be misleading, as it doesn't always signal menstruation. Some women spot during ovulation, which could be mistaken for a light period.
In that case, unprotected sex could certainly result in pregnancy. And whether pregnancy prevention is a concern or not, having sex without a condom or other type of barrier method still puts partners at risk for contracting sexually transmitted diseases. Safe sex, in other words, is the smartest sex no matter what time of the month it is.parents.com
Since we're already dispelling the myth that you can't get pregnant on your period, why stop at just one contraception misconception? Here's something else you should know:
Some women mistakenly think that urinating after unprotected intercourse will flush any lingering sperm from their reproductive systems. That's anatomically impossible since sperm enter through the vaginal opening whereas urine exits through the urethral opening between the clitoris and the vaginal opening.howstuffworks.com