Weeks along and how it's calculated?

I know they typically go by the first day of your last period, mine being January 17th which would make me 5 weeks 2 days. But how does that work when my period was 5 days long, and you typically don't ovulate for 14 days past your last day of your period? Even if I were to conceive directly on my highest peak day (per glow would be February 4th) wouldn't that make me barely 3 weeks along?  Sorry if I sound silly!