Life is not exact...
Maybe, if a cycle is longer than 31 days.
If a woman ovulates 2 weeks after the start of her last period, every cycle, then 3 weeks after the start of her last period would be too late to get pregnant. However, if she ovulates a few days later occasionally (maybe 2 weeks and 5 days) after the start of her period, then her egg may still be fertile at 21 days after her lmp. Days are estimates.