Supplementing is often a contentious, controversial subject among breastfeeding parents. Before my first child was born, I had done a lot of reading in preparation to breastfeed, much of it warning would-be nursing mothers against any sort of formula supplementation. Terms like "nipple confusion," and "refusing the breast" were introduced gravely and vociferously, usually in proximity to paragraphs singing the praises of breast milk and the benefits of breastfeeding. Advising against supplementation is not baseless — it can negatively affect a parents' ability to nurse — but, from my personal experience, there is supplementing advice that helped me breastfeed. I credit nursing my firstborn for 17 months in no small part to supplementing, and I credit nursing my second baby for 21 months (with no supplements) with the positive nursing experience I' enjoyed the first time around.
I truly believe painting the issue as "formula feeding versus breastfeeding" and, frankly, fetishizing "EBF" (exclusive breastfeeding) in some circles, can be counter-productive. The idea that nursing is all or nothing, or that breastfeeding can only look one particular way, is likely to dissuade discouraged parents who are coming at it from a different angle or experience. More importantly, it's disingenuous. You can breastfeed a baby who has had formula. You can breastfeed a baby who regularly receives formula, under the right circumstances. If you're like me, supplementing can be one of the many tips in your breastfeeding tool kit.
As with all things child related, there are absolutely no guarantees. Every nursing parent's body is different and every baby is different. As such, it's important to talk to your doctor/pediatrician/lactation consultant/etc about what is going to work for you. However, I can tell you what advice worked for me, because I believe we can learn from one another's experiences.
Don't Replace A Feeding With Formula Until Your Supply Is Well-Established
Among the risks one runs when supplementing is that doing so will affect your supply. This is a bigger risk if you replace scheduled feedings with a bottle of formula because, in essence, you're telling your body, "Hey, the baby hasn't eaten in a while. I guess they're good. Boobs, you can go ahead and ease up on the milk production."
This, in turn, diminishes your supply, which will frustrate your baby when they do go to the breast and don't get enough to satisfy or nourish them. So do make sure you're nursing frequently and for a sufficient period of time. This could mean you're nursing 10 times a day and for 20 minutes on each breast. It's time consuming as hell, but normal.
Provide Only Enough Formula To "Top Off" Your Baby
Your body needs time to figure out just how hard it has to work to feed your hungry kid. Some women need more time to get things going (especially first-time moms and women who have had a c-section, since their body is also working super hard at recovering from major abdominal surgery). In the meantime, your baby doesn't care about you or your recovery or capabilities. Nope, they just want to eat. (Though, hey, newsflash you selfish, greedy little babies: you still need to learn how to do this, too, so maybe you're being a less efficient than you could be.)
You can't blame them, of course. They need to eat pretty constantly. Supplementing with just a little bit of formula (seriously, in my case it was milliliters at points, though a few ounces will sometimes be necessary depending on the circumstances) after a feeding (in my case, anyway, but some people recommend before) will be enough to satisfy. Your body is still learning that it has to produce more milk, so your supply will (hopefully) improve, but until it does your baby isn't hungry or nursing so frequently it's stressing you out to a point that will hurt your supply.
Slow-flow nipples — bottle nipples that keep the milk from coming out too fast — are recommended for breastfed babies because they are more akin to human nipples in the speed with which your baby can get milk.
This does a couple of things. One, it keeps things more consistent for your baby. Two, it makes your baby less likely to prefer the bottle to the breast (which they might if they can get more food quickly as opposed to having to work for it, like they do when they're latched on to you or a slow-flow nipple). Talk to your pediatrician about what will work best for your little one's needs.
Add Pumping Sessions When Appropriate
Sometimes establishing supply is an issue, simply because baby is not inclined to nurse enough to get the dairy farm in full swing. (Full disclosure: this was not my experience.)
In cases like this (and others) doctors and lactation consultants will recommend regular pumping sessions, or even pumping after feedings, to ramp up production.
Once Your Supply Is Established, You Can Replace A Feeding (But Try Not To Do It Too Often Until Your Supply Is Super Established)
Often, supplementing parents will find they can slowly wean off formula all together. (Hooray for you, if that's what you want!) But sometimes parents will choose to continue incorporating formula into their baby's diet for any number of reasons. Replacing a nursing session with formula, while continuing to breastfeed, is best attempted once your supply is well established and you generally feel confident in your ability to nurse. After all, your body is always learning how much milk to make (and what kind, which is amazing), so replacing feedings too frequently can dwindle your supply.
That said, once your body gets the hang of things, old habits are hard to quit and you can usually play with your schedule or routine without deleterious effect. I personally didn't replace nursing sessions with formula until I was actually able to exclusively breastfeed for about two solid months, and then only infrequently for several months more (usually in an "emergency" situation, like if I hadn't pumped enough at work the day before so my partner could feed our baby while I was gone). Once I really had things down, I began combination feeding — breast milk whenever I was with my son but formula when I wasn't — starting around the time he was about 6 months old (replacing pumped breast milk with formula bottle by bottle) and then full-time when he was about 10 months old. We stopped giving him formula at 12 months, per his pediatrician, but I continued to nurse him until he was 17 months.
Nab Free Formula Samples Whenever Possible
Your hospital might have them, or you might even have been sent samples in the mail. Take advantage of free stuff whenever possible. That sh*t is expensive, dude!
I hated the idea of buying formula when I wasn't really using it, at least not full-time. My baby was taking so little at a time that figuring out the proper serving was difficult in those quantities. I wasted so much formula. If I'd paid for it I would have been really miffed. However, I wound up with so many free samples that, even with regular supplementing for a month or so and occasionally after that, I didn't pay for formula until my son was about 4 months old.