If you don't have a period, it means your body has not prepared your uterus for impregnation, therefore, you're not fertile. If you want to reproduce, you usually can find having a regular period as a good indicator of fertility, as your womb is hospitable for nurturing a fetus. HOWEVER, I have noticed that when you're unhealthy, and especially when you're overweight (usually the result of a SAD-extreme diet), you have extremely uncomfortable, heavy periods, and the thinner you get, the less heavy the periods are. Of course, once you lose too much of your body weight, you get to a point where you can't nurture a fetus, and your period stops, because your body no longer even tries to prepare itself for implantation, as the body is no longer an ideal host for gestation. The same thing happens when you get too overweight and unhealthy--you stop having a period and can't get pregnant because you're simply not healthy enough to carry a pregnancy to full term safely. Irregular periods would be a definite sign of ill-health, and no periods are a serious cause for concern. It's not natural. Your body knows whether you're healthy enough to carry a fetus for 9 months, and if you stop having your period, your body is telling you something. Simple as that. The purpose of existence is to reproduce, and while there's no need for anyone to force themselves to have children if they don't want them, if your body cannot produce a monthly period--ie: a healthy environment in which to gestate--you obviously have some malfunction in your body that needs attention. Probably nutritional deficiency.