Being underweight is not as common, especially if you are eating a healthy diet in general.  But of course it can happen, and it is far more prevalent in countries like France.  But chances are if you are thin, both your parents were also thin in general terms.  Some people naturally have higher metabolisms that keep them irregularly thin, and some of course suffer from eating disorders or otherwise.  But if you are genetically predisposed to be this thin, you can of course be underweight.  Frankly, from what I’ve found, you should not actually measure your weight by pure BMI.  Of course if you fall into the general range they have, it’s generally not a horrible thing.  But you need a BMI of at least 18.5 in relation to every factor.  You can’t determine it just by height in short.  For example, there is bone mass.  There are plenty of people who fall on the higher end of their BMI for height or otherwise, and though they would be considered nearing overweight or even overweight for height, their bone mass plays in and they may even be underweight.  In addition, there are some who are technically overweight, because they have a large mass of muscle in general.  Their body fat percentage however is extremely low to say the least.  So while usual standards are fairly stringent, you need to consider all the factors before judging yourself to be of normal weight or overweight.