How does water fasting affect menstrual periods?

greetings, i would like to start a long water-only fast (3 weeks or more) but i am concerned that this could interrupt my menstrual cycle. i am very overweight (need to lose 80 pounds) and wonder how a long water-only fast can affect a woman's menstrual pattern. i would like to hear from women--especially those who have done prolonged water fasts. has a long fast made your period go away? does prolonged Water Fasting affect fertility? all coments and information on this issue are very much appreciated.

thank you,
lovemywater