is it necessary to have heartbeat detected already?

I am from Pakistan and recieved positive test beta hcg test result on 14-Jan-2012. On 16-Jan-2012 I visited my gynecologist and she confirmed pregnancy. On 31-Jan-2012 i revisited dr. to discuss urine test result (that show WBC account 25-30 and dr advised for ultrasound of kidney. she prescribed antibiotic Amoxil for 5 days to control any infection). Unfortunately Dr. did not notice fetal heart beat in abdominal ultrasound (that should be in her opnion), therefore she asked me to wait for 1 more week and repeat ultrasound on 7-Feb-2012. Please note, previously my pregnancy ended on premature rapture of membrane on 3-July-2012 after complete bed rest of 7 months (vaginal bleeding in first 4 months) but gave birth live baby boy who did not make through and died after 11 days of his birth. Now this sudden news is very devasting for me. Please will you guide me on it, with respect of my LMP 5-Dec-2011, is it necessary to have heartbeat detected on 31-Jan-2012 in scan?