What is considered 6months in pregnancy?

Des • 2nd pregnancy due Feb17'! Back to back pregnancies- first baby will be 1yr old Jan 15 💗 #teamgirls
Why is pregnancy months different from actually counting 30days a month? Is 21 weeks 6 months pregnant or 22? Or 24? Lol everyone seems to tell me different. Open to explanations :) 
Ps: I'm 21w first baby