31 day cycle changed to 37 abruptly. trying to conceive.

This is confusing!: Cycles can change from time to time for no obvious reason. The trick here is pinpointing when you actually ovulate. You can do this by using ovulation indicator kits (urinate on a stick to identify hormone that goes up when you ovulate). An excellent resource on the subject is "Taking Charge of Your Fertility" by Toni Weschler. Good luck. LGromkoMD