Is it possible to get pregnant right after my period ends?
The answer to this question is yes, but to understand why we must examine some of the physiology behind conception.
The vagina is naturally too acidic and dry to support sperm survival and transportation. In order for fertilization to occur the woman needs to be producing cervical mucus that coats the cervix, alters the pH balance of the vagina, and provides a means for sperm transportation. Mucus is typically created for 3-7 days leading up to ovulation. While a typical cycle length is between 21-35 days, cycles on the shorter end of the spectrum (21-25 days) typically place ovulation between days 9-13 of the cycle. In these cases, the onset of fertile cervical mucus may begin immediately (or even during) menstruation. For example, you could have your period for cycle days 1-7, but begin producing fertile cervical mucus on day 6 leading to day 10, with ovulation occurring on day 11. Since sperm can live up to five days in fertile mucus conditions, an occurrence of sex or insemination during the end of the period or for the days following menstruation could result in a pregnancy.
If you chart your cycles and they tend to be on the longer side (over 35 days in length with ovulation never occurring before day 20 or so) then there are likely a few days after menstruation that cannot lead to pregnancy. However, even people with "regular" cycles (around 28 days) can switch to having short cycles (less than 25 days) unexpectedly.
Be aware that all bleeds are not necessarily periods, either. Bleeding can occur during ovulation, during pregnancy implantation, after an anovulatory cycle, after particularly rigorous sex, or due to a health condition like endometriosis or cervical polyps.
The only way to know if you are having a period and whether you are within your fertile window after menstruation is to chart your cervical mucus. This takes practice and education. See the references on this site for more information, read Taking Charge of Your Fertility, the Justisse User Guide, or seek professional assistance from a Fertility Awareness Educator.