Thats a difficult one @Deborah8 - you need to discover what's behind them finding themselves in that situation that necessitates the requirement for short-term 10 week accomadation where they baby will be born during that time. Normally, an extended family would assist if they were in a position to. Or a property let agency with short-term properties available to include a deposit etc - although they may not wish to have to set up seperate utility bills etc for a short-term rental and may view ABB as a means to avoid that - but of course it would be at a cost to you if they are in the home full-time utilising all of your facilities.
You also need to consider what @Donna15 say's about potential medical emergency and a screaming new born in your home - plus the issue around what happens if they do not find alternative accomadation after 10 /12 weeks. Tenancy rights kick-in after 12 weeks, I believe.
I don't know if your guests are Brits or from outside of the UK but there have been stories (and I'm not sure how true they are) about couples (and I think there are terms for it 'passport babies' or 'tourism babies') who purposefully decide to have their child born in the UK (or elsewhere) as the child becomes a national with all the rights that, that entails.
So try to establish exactly why they are in this predicament first - and factoring in all issues raised, decide whether its right for you.
Personally, I wouldn't take the booking - but then, I only rent for a maximum of 14 days to holiday guests, but most often only to weekenders and business guests - but that's because it suits my preferences.
