regarding tiffi scan

My wife 5 months pregnant 20 days back we went for doctor check up before that she had mild stomach ache later doctor referred for scan it was perfect baby very healthy with gud organs even spine , skull everything gud then again doctor gave medicine for 20 days later after 20 days we went for general check up than doctor said that belly has nt developed much wen compare to previous check up so she gave medicine for 10 days and referred tiffa scan and triple mark test What May be the reason behind this Wheather baby is nt gud or any other reason Please let me know we are very scared