What length should your Luteal phase be and how can it be lengthened?

Lisa • Married 5 years to a wonderful man. Son born 26 Jan 15. Expecting another boy in Nov 16 X
Anyone got any ideas?
​Mine seem to be 9/10days and internet research is conflicting on whether this is an issue. 
​What can a women do to length the LP?
​Anyone had any advice from a doctor?
​Thanks ladies xxx