Before you go back to work, you should be sure to have 3 daycare days worth of milk in your freezer to act as a cushion.
Once you start working, you will be pumping for your baby, not for your freezer.
So, let’s say your baby eats 12 oz of milk every day at daycare (expect him to eat about 1 oz – 1.5 oz for every hour he is at daycare). On Sunday night, you will take 12 oz from your freezer stash to defrost for Monday. On Monday, whatever milk you pump you will send to daycare for your baby to drink on Tuesday. Tuesday’s milk is for Wednesday, Wednesday’s milk is for Thursday, Thursday’s milk is for Friday. Whatever you pump on Friday should go back in the freezer.
The trick? Whatever you put back in the freezer on Friday night should be equivalent to what you took out the Sunday prior (in this example, 12 oz). If you come up short, you should do some extra pumping over the weekend to make up the difference.
Fresh milk is preferable to frozen milk, so it doesn’t make sense to freeze fresh milk and defrost frozen milk. Use whatever fresh milk you have first!