Not necessarily but it is entirely possible. Everyone is different, but I always weigh heavy during my period due to water retention. I am on mine now actually, and when it started on Wed I was weighing in like 6 lbs heavier than what I weighed at check-in on the previous Saturday. It bummed me out at the time, but now that I am halfway through, I am within 2 lbs of last weigh-in, and frankly I increased my weight-lifting this week so I would not be surprised if I don't lose any weight this week. Tomorrow is weigh and measure day so we shall see! Just don't be too disappointed if the scale doesn't show a loss this week. Usually I make up for it the next week, so if you have been eating right and exercising, you should too.