18 is underweight. Whether underweight enough to cause problems with your period depends entirely on your own physiology.

BMI isn't foolproof, it is just a guideline. It doesn't take into account muscle mass, etc..

Loss of your period is caused by lack of enough body fat to stimulate hormones, by too much exercise, too much stress, etc.. A lot of things cause it.

If you get to the point where you stop having it for three months or more, you need to see a doctor/gynecologist - whether or not you are currently willing to seek help for your eating disorder. The longer you go without your period, the higher the risk for infertility and osteoporosis (estrogen helps absorb calcium into the bones).

Any thing below 20 is pretty much bad. Your body is telling you that you need better nutrition. it took a very long time (about 2 years) for my period to come back after i was back to a BMI of 21 and it's all to do with hormone levels. You should try really hard to maintain a weight above 18 - i know it's difficult but you will have a lot more energy and your moods will be more stable. hope this helps.

I only got mine back last week after 6 months and that was only after surpassing my healthy weight. It might not be lack of body fat necessarily, as there are other factors suck as too much exercise and such...
I would agree with Katie, def. keep it up a bit so that your body can heal and regulate.
Good luck :)

About 2 1/2 years ago I started to exercise and eat better and dropped about 10 pounds in a few months. My period started to get irregular about about 2 months and then just stopped for a year and half. I tried herbal supplements, but then finally turned to birth control. My doctor said I could be exercising my estrogen away and also that I was stressed. I was also down to a BMI of 14%. So I think everything contributed to it. I'm still on the pill now after about a year and hopefully off soon now that it's regular again. Hope this helps

I recently went through a bit of relapse, and lost about 1.5 stone in a month or so... ive stabalised my weight now and am not underweight but have not had my period since (4/5 months)... Im going to go to the doctor as soon as i can as i know it cannot be a good thing and would recomend you do to.