During 1st stage of labour, FHR should be auscultated in low risk pregnancy every:

FHR along with its rhythm and intensity should be noted every half hour in the first stage and every 15 minutes in the second stage following rupture of membranes.To be of value the observation should be made immediately following uterine contraction,the count should be made for 60seconds.Normal FHR ranges from 110-150/min D.C.DUTTA'S TEXTBOOK OF OBSTETRICS,pg no:135,7th edition