babies not growing well


I'm 31 weeks, went to the Dr on Monday and the babies are moving well, with good heart rates but they are not growing as well as they would like. They still are growing, but not much, baby b is not even 3 lbs and is under the 1st percentile. Dr isn't too concerned but I will start twice weekly non stress tests next week and at any time they could decide it's time for me to deliver. Has anyone else had this happen??