It can be scary and disheartening when your breastfed baby isn't gaining enough weight. Fortunately, there are many ways to fix this situation and get you back to exclusively breastfeeding!
If your breastfed baby isn't gaining enough weight in the first 6 weeks, usually it's due to not transferring milk well enough and then your milk supply may not fully come in. But the good news is that this early on you can likely increase your supply pretty easily. You'll also have to get baby to transfer more effectively - see my sleepy baby post for tips there!
You start by doing triple feeds. You will likely pump enough to supplement with your own milk. If not then you may need to do
Offer both sides every feeding. Limit to 20 minutes per side if you're going to pump afterwards so that feeding sessions don't take hours.
Pump after feeds as often as possible for 20 minutes, double pumping, make sure you have well fitting flanges and good pump settings (ask a lactation consultant to optimize these; I'll do a post in a few weeks on this too!)
For now offer supplement after every feed, 1-2oz each time. Use the milk from your previous pumping session (i.e. if you nurse/pump at 12, then nurse/pump at 3 and supplement the milk you pumped at 12). If you don't have enough milk to supplement 1-2oz each feeding, use formula or donor milk.
This way you can continue to breastfeed and get your baby as much of your own breast milk as possible, while increasing your supply and getting your baby to gain enough weight!
The next step will be figuring out when to wean off the triple feeds... triple feeding is not a permanent solution! I'll post about that soon.