What is normal when it comes to pumping output and changes in pumping output?
Most moms who are nursing full-time are able to pump around 1/2 to 2 ounces total (for both breasts) per pumping session. Moms who pump more milk per session may have an oversupply of milk, or may respond better than average to the pump, or may have been able to increase pump output with practice. Many moms think that they should be able to pump 4-8 ounces per pumping session, but even 4 ounces is an unusually large pumping output.
It is quite normal to need to pump 2-3 times to get enough milk for one feeding for baby (remember that the pump cannot get as much milk as a baby who nurses effectively).
Many moms are able to pump more milk per session when they are separated from baby. Milk pumped when you are nursing full-time is "extra" milk -- over and beyond what baby needs. Don't get discouraged if you are trying to build up a freezer stash when nursing full time and don't get much milk per pumping session -- this is perfectly normal and expected.
It is very common to have more milk than baby needs in the early weeks, which regulates down to baby's needs over the first few weeks or months. When your milk supply regulates (this change may occur either gradually or rather suddenly), it is normal for pumping output to decrease. For moms who have oversupply, this change often occurs later (6-9+ months postpartum rather than 6-12 weeks).
It is normal for pumping output to vary from session to session and day to day. Having an occasional low volume day is not unusual.
During a growth spurt, don't be surprised if baby drinks more expressed milk than usual, making it harder for mom to provide enough expressed milk. Growth spurts are temporary - try increasing nursing and adding a pumping session or two at home until the growth spurt is over.
Menstruation or ovulation can result in a temporary drop in milk supply. You might also notice cyclical dips in milk supply before your period returns, as your body begins the return to fertility. Hormonal changes also cause milk supply to decrease during pregnancy.
Remember that the amount of milk that you pump is not a measure of your milk supply!