Media nowadays generally give a misrepresentation of the perfect figure of women. It has now come so far, that the minimum healthy BMI value of 18.5 is considered by many women as the perfect body weight. Under the influence of the media, many women seek thereby to a body weight that is not healthy.
The problem is that your body can not function at such low values completely. To be healthy, a woman should have a normal heart rate and blood pressure and that includes a healthy BMI value.
To determine the ideal weight, you need not to focus on the desire to be thin, but you have to concentrate on your health. The most healthy weight for you is perhaps not as thin as you would like.