Bugs - have you been sick or off your food at all? It doesn't take much to lose (or gain for that matter!) half a stone, so I really wouldn't worry.
I was doing Slimming World when I fell pg with ds (I was a consultant), and wasn't yet at the weight I was aiming for. I continued to lose weight for a few more weeks once pg - even though I was eating a really healthy balanced diet.
However, this all changed later on in the pg, as I was confined to 'sofa & daytime TV' rest! I think I put on about 4st all told by the time ds was born!
Keep an eye on it, and mention it to the MW, but I'm sure it is v.common. Your body will ensure the baby has all it needs, so you don't need to worry about it going without.
The only time I would say worry, is if you were already at the lowest recommended weight for your height. Any idea if you are?
Hope that helps a bit