I don't know the causes but though it's early, it's normal (I was told). DD2 here got her first 2 teeth at 4 months and on a similar schedule to you, had her first set of molars in by just after a year. And almost always they came in 2 or 3 at a time (ACK!!). She is 21 months now with 3 of the "2 year" molars done. DD1 was way later, so it freaked me out! Hang in there! I am excited for her pain to be almost done!
4/26/12 at 5:43am