That depends on how much you weigh now, how much you have to lose. If you only have 4kg to lose and work very hard at weights it is theoretically possible you could end up the same weight but much smaller and more toned. If you have more than a few kg to lose it's unlikely that you wouldn't lose weight too. It would be very hard for a woman to gain 20kg of muscle for example.

Weight is not a problem, excess fat is. Being 80kg with a bodyfat % of 20% is incredibly healthy. Bing 81kg with a bodyfat % of 35% is not. The number of kg isn't an issue, only the composition is.

If you're doing WW i'd suggest you tell the leader you're engaging in a resistance-based exercise programme and ask them to either measure your bodyfat (there are a number of ways to do it but it needs to be done in the same way by the same person each time because the margin of error can be broad if they mix methods/measurers) or use a tape measure to take bust, waist, hip and upper arm and thigh measurements to monitor your loss as well as weighing you - they will show the real situation when the scales aren't. Stay focused on your goal, to lose FAT, not just weight.

Muscles need water to repair themselves. This can be slight or marked. For example the day after a long run (15-20km) i can weigh as much as 4kg extra, my muscles grab as much water as possible to repair themselves, but it comes off over the following few days. These things need to be borne in mind if you're doing a difficult weights session or running a long way the day before a weigh-in.

Bx