Most BF babies lose weight in the first two weeks and they can lose up to 10%, it's normal. Also normal for them to regain that weight in fits and starts, hence why the NHS advises midwives to only weigh at day 5, 10 and possibly day 14, weighing them every couple of days isn't needed and can cause undue alarm. My youngest only lost 5% by day 5 however after that he didn't gain a gram until he was 10 days old, he then gained 1 and a bit oz a day so regained his birthweight by 16 days. My in laws exclusively formula feed and my youngest sister in law took 3 weeks to gain back her birthweight despite being on formula completely, so formula is no guarantee they will gain better xx