Menstrual Irregularities

Here's a page which illustrates/explains different types of menstrual irregularities:

http://www.femalehealthmadesimple.com/FileTwelveFinal.html

"Any woman of childbearing age who has missed more than two consecutive periods -- but was not pregnant -- needs to investigate it and possibly have her period induced. Preventing the development of hyperplasia is the reason."
From: Endometrial Hyperplasia, WebMD

"Excessive estrogen stimulation can lead to significant increases in uterine lining thickness. The tissue becomes hyperplastic or atypically hyperplastic . It may become cancerous."

(and)

"Inadequate ovulation may cause excessive, irregular, or frequent menses recurring less than every 3 weeks. Inadequate ovulation leads to an excess of estrogen, which stimulates growth of the menstrual lining, and a decreased level of progesterone - which normally would be present in levels that would adequately thin the menstrual lining and prevent excess or irregular bleeding."
From: Dr Decker, Heavy Menstrual Cycles Management Options

"We like to see a lining of at least 8mm in thickness when measured by ultrasound at the time of maximal thickness during the cycle (see above ultrasound images of an 11.5 mm lining). There is some ongoing debate as to "how thin is too thin", as well as to "how thick is too thick". In general, 8-13 mm is good, less than 6 is potentially a problem, and greater than 15 or so could possibly reduce chances for a successful pregnancy."
From: Uterine problems causing infertility or miscarriage