Never ask a woman her age or weight or so goes a maxim. I am not sure why women are singled out for this as many men I know would be equally uncomfortable sharing their vitals including weight. Just add 30 pounds to however much you think I weigh is likely to be the most honest answer you can get. Your weight most likely changes everyday even if you are an extremely disciplined eater. Often this is just simple water retention and may not even have been caused by that black forest cake you tucked into when no one was looking.
Your body weight is a summary of the weight of your bones, body fat and muscle. For an average person, bones comprise approximately fifteen percent of your body weight. Unless you are not absorbing calcium for some reason, the mass and weight of your bones are unlikely to change. It is for this reason that if you are watching what you eat, it is important to ensure that you are getting enough Calcium. Exercise, especially vigorous exercise like weight training also helps in strengthening bones. Some people believe that they have heavier bones but in the calculation of your weight range, this is already factored in and if you are outside your ideal range, its not because of your bone frame.
The mass of body fat and muscle is not the same and therefore depending on how much body fat and lean muscle you have, the shape of your body and your total weight can vary greatly. Therefore a person who weight 160 pounds with say 35% body fat would look very different from when he weighs 160 pounds but has only 8% body fat. This is because the body fat has been replaced with dense muscle making him look muscular and lean instead of a showing off a beer belly. The weight can remain the same because of the heavier dense muscle.This illustrates that it far more important to understand and track your body fat rather than your body weight. Other good indicators of the quality of your weight include your waist size. There are weighing scales that can tell you your body fat percentage. It does this by passing an electric pulse through your body which measures that amount of body fat. Doctors believe this is not the most accurate and if you are obsessive about accuracy, a visit to the clinic is required where they will measure the fold of your skin and then derive your body fat. Many people are what is known as skinny fat which means that the frame of their body looks slim but it hides a high body fat percentage. Think of this as Laurel and Hardy both weighing the same. The outcome can be very different.