SK,
EDD will be more accurate after bb scan by the gynae.. it will be kept adjusting until more or less fixed... ur case, shd be 15th to 19th, definately not 1st..
'cos every women's period cycle is different, it varries from 28 days cycle to 35 days cycle. if you have not known ur cycle, they will based on standard cycle which is 28days.. hence, in this case, not so accurate if based on LMP.