Please explain

Can someone please explain to my hard headed husband the weeks/months during pregnancy. 😂 A little background information, I found out I was pregnant Jan. 4th, and my last period was Dec 9th, 2020 and about two or three days ago of this year, Jan I went for a check up(I already knew I was pregnant, but needed to go to the doctor anyways and get checked.) so they told me, I was 6 weeks pregnant, I told my husband and he doesn’t understand how I’m 6 weeks, if he says “6 weeks ago you were on your period”, can someone explain to him, ‘cause when I do, he gets confused lol