The fertile days are those in which, by having sexual intercourse with penetration, an egg and a sperm can unite and fertilise.
We must bear in mind that the egg has 24 hours to fertilise from the moment of ovulation. On the other hand, sperm remain active in the female reproductive system for 48 hours, and some studies have even determined that they can remain active for up to 5 days. We also know that ovulation occurs about 14 days before the next period.
With all this data, if we start to calculate, people who have 28-day cycles, ovulation will occur on day 14 of the cycle. If we have intercourse on day 9 of the cycle, the sperm can act up to 5 days after ejaculation, being day 14 of the cycle and ovulation occurs. So day 9 can be a fertile day.
Let us now imagine that a person has shorter cycles or that, due to a specific irregularity, he/she ovulates earlier. In these cases, the fertile days could be before day 9, thus coinciding with days that are still menstruating.
The implantation of the embryo in the endometrium occurs 6-7 days after fertilisation, so it could even occur 12 days after intercourse. Even if you have had intercourse during your period, once this period has passed, there is no bleeding at the time of implantation, and if the endometrium is receptive, pregnancy can occur.
In conclusion, fertile days can coincide with menstruation. Even if we have normal or long regular cycles, ovulation can always occur earlier than expected. Therefore, if pregnancy is not desired, contraception should also be used during menstruation.