When should I tell work?

Im 6w3d have my first appointment on New Year's Eve but can't stop wondering when it's okay to tell work? I work in cosmetics and only occasionally have to lift some things but could be up for a more permanent position at a counter soon (I'm on call currently) so I don't want to ruin my chances by telling to soon and having them not wanting to hire someone who will need to eventually take maternity leave. Stressing over this!