Well that's complicated. A LOT of people who can get pregnant don't have regular cycles, don't have predictable ovulation, don't know or have all the regular signs of ovulation, etc, that make tracking fertile periods tough. They are also people who bleed during ovulation, or have sporadic bleeding throughout their cycle, or bleed frequently from sex, all of which can confound predicting ovulation.

To compound that, sperm can live up to 5 days in the receivers reproductive tract extending a typical ovulation period to 11 days. https://www.webmd.com/infertility-and-reproduction/sperm-and-semen-faq