Figuring out months! 😂

Kaytlyn • 28. Happily married. PCOS. Mommy to 4 sweet babies JCJ 7/26/14 KRP 6/10/16 ARP 1/17/20 EMP 11/11/21 💙💙💙💜

Someone help me understand... I like to know how many weeks my son is while he’s still little, but I also am confused on figuring out the months. My son will be 20 weeks old this coming Friday so that’s 5 months, but he was born on the 17th so if I go by the date he will already be 5.5 months... which do I go by?! I’m dumb, I know lol