Can someone explain the +4 to me?

N • Beautiful Nikolai born 12/29/17 👶💙 Lucca born 1/13/21🌻 In love and feeling myself 👨‍👩‍👦‍👦

I always see 24+4, 30+4, etc. What exactly does that mean? I am super curious :) Thank you very much! I am currently 6 weeks and 5 days according to my OB (before my first ultrasound) and I am also wondering how that applies to me.

EDIT* Thank you all! I was thinking it might be the day or possibly something weird like the corrected date XD That helps a lot, I appreciate it!