I am often confused when people refer to their baby's age in weeks. A friend posted on facebook "Wow my ds is 27 weeks! Time is going so fast!" I recently asked a lady on the bus how old her dd was and she said "13 weeks."
Given that babies born at 37 weeks and those born at 42 weeks are all considered full term it seems pedantic to state a baby's age in weeks.
AIBU to think that once your baby is over 1 month old you can start refering to their age in months even if you say "3 and a half months" or "nearly 2 months." Is there a reason why the exact age in weeks is important? Maybe it is just me and my bad maths skills that stuggle to work out how old the baby actually is.