Well, as a fyi, how much do you fast, how long, and when it did start, did you eat anything? I'm not sure starting a 18 labor on zero fuel is a good idea..
I figure that any time after 2 weeks before the due date, the baby is "done," so if I feel the urge, I stop eating. If I'm not having contractions after 24 hours, I eat for a day, and then skip again.
When contractions start, I'll let them go 8-10 hours, and then eat. If the contractions stop, then (obviously) we're not having the baby yet. Don't eat until contractions start again, give it some time, etc. Eventually, the contractions don't stop even after eating.
I've never had any problem with needing "fuel" for labor. After all, I've been storing fat for 9 months!