They are early atrial contractions. They may be significant and associated with congenital heart disease, but they may not be. Generally they are well tolerated in children and adults. I would suggest a fetal echocardiogram to exclude the possibility of any congenital anomalies or malformations, and if negative then reassurance that it is generally an innocuous finding.
i have lots pacs, not sure in a baby but my cardio says they are nothing , go on the pg form on here and ask if anyone on there has them, but your dr should know what to do.