too much exercise ( maybe even stress ? ) can cause fully adult women to temporarily stop having periods.  the conditions have to be right for pregnancy.  if you're destined to lose the child it is better not to get pregnant in the first place as far as evolution is concerned - better to save resources until the time is right.
 

 @Dissident Sound @pipe @finrod @StephanGuy 

That's victim mentality. Resources should be so plentiful that a woman can lose 20 babies before having one survive to birth.