Why do moms say their baby's age in months?

Thats a good question...It might be because when we are pregnant going by weeks is more important then going by months, cause each week you are pregnant makes a BIG milestone in the baby's development, a matter of a day is a big difference in a developing baby....So maybe after the baby is born we are already used to counting there age by that...at about one years old for my son though I began saying 1 year and 3 months..