I have to contradict the advice given to the previous poster in this particular situation: it *could* be important to wake a baby who is not gaining weight and may be below the 5th percentile. Many moms set alarms to wake themselves, and then baby, to nurse. We don't know enough about this baby/mom to say "never wake a sleeping baby". This baby is probably okay, but a baby who is "failing to thrive" might not be able to wake up enough to signal to mom that s/he needs to nurse.
THE EASIEST WAY TO TELL IF A BABY IS NURSING ENOUGH IS TO COUNT WET/POOPY DIAPERS. If she is having 4-6 wet and 1-2 dirty diapers (at least a quarter-sized BM) in 24 hours, then she is probably getting enough milk. You said your daughter is gaining weight, so it may just be that she is small and that she will always be in the 5th percentile.
If her doctor wants you to supplement, ask why. Most docs will say "failure to thrive" - but ftt has more than a weight component. Is she alert or lethargic? Rolling over? Trying to sit up yet? Using her arms a lot? Tracking with her eyes?
What does your mama gut say? You don't think three feedings a day is enough- is she eating other foods that may be filling her up?
Most babies at 7 months of age will breastfeed 6-8 times in 24 hours- but some may only breastfeed 4-6 times and that might be normal for them. Have you kept a log for a day to see how many times you're nursing? Sometimes it's more often than you think! And while i would normally say, 'Watch your baby, not the clock', you may want to make note of how long she breastfeeds. Write down any solids she eats, too.
Are you supplementing with formula at all?
Is she getting bottles of water or juice?
And finally, were you yourself a small baby?
There are growth charts specifically for breastfed babies- and it is important for docs to know this- but they only apply to exclusively breastfed babies. You can check them out here: http://www.kellymom.com/babyconcerns/growth/growthcharts....
http://www.llli.org/FAQ/growth.html
I hope this helps. You child may simply be what we call a turbo nurser and gets what she needs from you very quickly!
I would recommend that you call a La Leche League Leader or get to a meeting: http://llli.org/WebUS.html
Wishing you & your baby girl the very best!
Naturally, J.