Sadly some professionals fixate so much on early weight loss (which is normal) that they encourage mums to switch to formula unnecessarily.
Exactly this.
When my baby was born she weighed 6lb 11oz. My milk came in on day 2.
At 5 days she was weighed at home and was 6lb exact (by those scales) which is bang on 10% weight loss pretty much, but she was feeding lots so I knew she was fine and I was producing plenty so I told them I was going to carry on breastfeeding (actually I think by then they knew I was adamant about breastfeeding so didn't even suggest topping up or anything, they knew they'd be told where to go
)
She didn't get back to birth weight til she was over 15 days old (was 6lb 10oz at 15 days). But after that she gained an average of 8-10oz a week and gained just under 2lb during the two weeks over Christmas and New Year (around week 5 to 7).
Also if it seems your baby has lost a lot of weight, please bare in mind that different scales may be calibrated differently. Just being out by a few ounces makes a huge difference to a baby's weight.
If you have any concerns with weight, make sure they are feeding efficiently, listen for swallowing, try making sure they feed as long as possible from one breast, don't let them go too long between feeds (if they are sleepy, wake them every couple of hours at least and feed them), likewise, don't schedule feeds, feed as often as they show interest - don't wait till they cry, sucking fists, grunting, moving their head side to side 'rooting' etc are all signs of hunger. Last of all, if they are producing plenty of wet and dirty nappies you know they are getting enough.
If you're still worried, contact a peer supporter