Just a few twin Q

Mommy2four

Okay, i feel silly asking this but i really don't know-

Do twins grow at the same pace as a singleton, in utero? If so, is there a time they slow down OR they are just born earlier so they are smaller?

I have a friend with twins,now two yrs. and when they were under one she would always say, "they are 8 months but only 6 months adjusted". Do twins always or most of the time develop slower in the first year?