Supplementing on a daily basis
If you plan to supplement with a bottle of either breast milk or formula on a daily basis, be consistent about the time of day when you supplement. For example, if you decide to supplement at the late afternoon feeding, you should try to stick to this schedule. Varying the times of day when you breastfeed and supplement will wreak havoc on your milk supply, leaving you with uncomfortably full breasts at certain times and a small milk supply at others.

When you start supplementing with a bottle, introduce the supplemental feeding once a day for about a week. If you need to miss more than one feeding a day, you should introduce the additional supplemental feed during the second week—after you and your baby have adjusted to the first supplemental feeding.

If you plan to feed only breast milk while supplementing routinely
If you plan to feed your child only breast milk as the supplemental feeding, you will need to express your milk while you are away to help prevent engorgement and to maintain your milk supply. If you leave your baby for longer than 4 to 5 hours for 3 or more times a week and do not express your milk, your milk supply will probably diminish.

If your baby is less than 4 months old, you will probably need to express breast milk every 2.5 to 3 hours while away. If your baby is 4 to 7 months old, you will typically need to express milk often enough so that between expressing and nursing, your breasts are emptied 7 times every 24 hours. If your baby is 8 months or older, how often you express milk will depend partly on how much solid food the baby is eating and how often the baby nurses.

See Milk Expression for more information about techniques and milk storage.

If you plan to use formula while supplementing routinely
If you choose formula for supplemental feeding, express only enough milk to relieve pain or pressure. Your breast milk supply will eventually adjust to a more limited breastfeeding schedule.

Supplementing occasionally
If you need to supplement only occasionally—for a night out, for instance—you can express your milk earlier in the day, or prepare a bottle of formula in advance. If your breasts are uncomfortably full when you return home and your baby is not ready to be fed, express your milk and save it for later use.