The others are right, you don't owe them anything...

But, ethically what do you think is the right thing to do?

If you don't tell them till 12 weeks, or until your probationary period is up, they are still under no obligation to hold your position for 12 months (or any period of time) while you take maternity leave - you have to have been there a year to have that or the company would have to have a unique policy in place. Unless you're planning on returning to work within a month of two of having the baby, then chances are they may not keep your job open for you, especially if they think you've withheld something from them or been dishonest.

Which brings me to my next point. If I offered somebody a job and they told me then and there that they were pregnant and were planning on taking X amount of time off when the baby was born, I would respect them for their honesty and integrity right away. Providing I could accomodate their needs (ie probably less than 3 months maternity leave) and felt they were right for the job (more likely if they've been honest with me) I would still hire them because I would be thinking longer term.

So I guess it comes down to what you want out of this job - are you thinking it is a long-term proposition and somewhere you'd like to work after having the baby, or are you thinking it will be good for the next 6 months. If it is the latter, then don't tell them. But if it is the first one, I think you owe it to them (and you) to be upfront and honest. But be aware, if you're wanting to take 12 months maternity leave it could be an issue, whichever way you go.

HTH it's just my opinion for what its' worth.