It won't impact your supply. But, like someone else said, YOU will be uncomfortable. I went to a conference once for the same amount of time. I had no time to pump or express while I was there... I was in some pain! My breasts were huge. Good thing I had a suit on that hid my breasts pretty well. I came home and pumped right away-- knowing of course, that I had another 3 hours to feed my lo. So, make sure that when you pump that you leave enough time, approximately 3 hrs., to replenish your supply. For me, 2 hours was too little time.