Is it better to introduce formula while my daughter is still drinking breast milk? If so, how gradually should it be introduced?

Question

My wife was fortunate enough to produce a lot of breast milk (enough to feed our 7-month-old daughter and also pump a significant amount, which we froze). In the first few months of our daughter's life, she was exclusively breast feeding, but now my wife is back to work and her supply is naturally dwindling and she's now at the point where she's no longer pumping. We're guessing that on an average day, our daughter gets maybe 4-6 oz. per day from breast feeding, and about 22-24 oz. from our frozen supply.

We probably have about 6 weeks' worth of frozen milk left (2 months' worth at the most), and at that point, my wife probably won't be producing anything, so we'll have to transition to formula. Note that the milk is in a deep freezer, and it's unlikely that any will be more than 6 months' old when she drinks it.

My questions are:

1. It seems better to me that we should start feeding her formula while she's also drinking breast milk, so it's not a big shock (and so we have breast milk to feed her in case we need to try different formulas, etc.). Is this true?
2. If so, is there an ideal time to start the formula (e.g. when we have 2 weeks' worth of breast milk left)? At that point, how much formula vs. breast milk should we use? Should we introduce the formula slowly (e.g. start with 1 bottle per day of formula vs. 4 of breast milk, and then work our way up to 5 formula?)
3. Should we mix the formula with breast milk (i.e. in the same bottle?) If so, similar question to above - should we start with a low % of formula in each bottle and work our way up to 100% formula in a bottle as a way to ease her into it?

Answer 1

I would personally take the direction of introducing formula slowly, partly to extend the time that she is getting any breastmilk (and the antibodies, etc. that it contains) and partly just because it sounds like it would make you more comfortable, and might make her less resistant or stressed by the change.

On average, combo feeding (both breastmilk and formula, and/or both bottle feeding and nursing) is well tolerated by babies, and aside from some possible changes in bowel habits, you aren't likely to see any negative impact of adding or switching to formula. If she does seem constipated, gassy, colicky, etc, for more than a few days or her discomfort is severe, consult her doctor.

You can start the switch as soon as you like, and go as gradually as you wish, but it's not recommended to mix breastmilk and formula in the same bottle because of their different storage and handling requirements. It can lead to wasting more breastmilk, and I think ideally you want to maximize how much of the milk your wife worked hard to pump actually goes into your little one's belly. If your daughter rejects the taste of formula, and offering it at different temperatures (warm, room temp, cool) doesn't resolve that, you may want to try the gradually increasing mix method, but I wouldn't recommend it as the best way to start.

Other things to keep in mind: If your wife currently pumps at work and nurses while at home, she can keep doing that probably as long as she wishes. She can even stop pumping at work, provided she does so slowly to avoid mastitis and other problems. Even if she isn't pumping a large amount of milk, it's still important to reduce pumping time and number of sessions gradually so she has time to adjust.

Her body CAN adjust to making milk just at night for the baby, if this is desired. This is how I handled my son's feeding once I stopped responding to the pump: I gave him formula at daycare, gradually stopped pumping during the day, but was able to feed him by nursing when we were together (morning and overnight, and most of his weekend milk needs as well). I have maintained this part-time supply for almost a year now, although in decreasing amounts as he enters his toddler years and relies more on solid foods rather than milk.