If you conceived on 5th December, then your do not get period after that and 19th December never be your LMP. You mistake it by spotting/ light vaginal bleeding
on 19th December which is quite common in early week of pregnancy
. Here your probable LMP was 19th-21st November in case of 28-30 days cycle.
So your expected due date for delivery will be 26th-28th August 2013. You can undergo one ultrasound
scan for better evaluation of gestational age
Be well & take care.