You can carry a newborn in a MT, although a wrap or ring sling is usually preferable. Depending on the height of the MT, you may need to fold up the bottom before tying the waist straps. If the baby is ok with legs froggied in the MT, then that is ok (of course checking for good positioning with knees higher than bottom), or they can be legs out if you cinch the bottom with a hair tie or something similar. Some MTs come with snaps or other way to adjust the bottom in for little babies. Make sure that you tie the straps behind their back instead of under their bum.