according to this article from about.com women are almost always fertile....

[Guest guest]

excerpt from a very misleading article:

link is posted below

"For some women with irregular cycles there is hardly a day in the menstrual cycle when they are NOT potentially fertile. The window of fertility is most unpredictable for teenagers and women approaching menopause. Thyroid disease, which frequently causes irregular cycles, can also cause unpredictable fertility. Textbooks also usually say that women are fertile for several days after ovulation. Dr. Wilcox of NIH's National Institute of Environmental Health Sciences (NIEHS) has studied fertility extensively. "

http://thyroid.about.com/od/hormonepregnantmenopause1/a/fertility.htm?nl=1