Family history should be taken into consideration in any adolescent with primary amenorrhea, as mothers who started to menstruate late will often have daughters who also menstruate late.
The human body, when taxed by extra weight (or too little weight; many severely underweight women do not even menstruate), does not get pregnant as easily.
For no apparent reason, a few women menstruate (with ovulation occurring) on a regular schedule as infrequently as once every two months.
Girls who have begun to menstruate and have missed three periods should be evaluated for secondary amenorrhea.
Although women may assume they cannot get pregnant if they do not menstruate, they may still be ovulating.