Ramki, Write a case statement...
case when [Birth Weight] < '750 gram' then '1-A'
when when [Birth Weight] between '751 gram' and '1000 gram' then '1-B'
when [Birth Weight] between '1001 gram' and '1500 gram' then '2'
when [Birth Weight] between '1501 gram' and '2500 gram' then '3'
when [Birth Weight] > '2501 gram' then '4'
end