Slow gainer

My little princess is 4 months old and only 11.4oz she was born 4 weeks early at 6.04oz she tends to put on between 2-6oz a week apart from once not putting on anything and last week putting on a ib my hv now has me worried as she said she's not putting on enough (just because of the 1ib gain) has anyone else had a slow gainer and should I really be worried she's happy healthy and lively and hitting milestones