Jenn,
Having worked for many years with various bosses, I don't think as workers we owe them anything!!!
They don't really care about us, as long as the work is being done...
If I were you I'd not tell them until after the 3 month probationary period.
If you do fall pg during the first few months & have not been their 12 months, then usually they don't need to keep your job for you either...
When my DH gopt his visa he took any job he could & for 3 months worked hard there & felt like he owed them something, but someone called & offered him work at almost $5 more per hour, at first he was too nervous to take it & put the current boss out, but when I eventually persuaded him to, the Boss was nice & said I don't blame you!
I know my work would chuck us out if they had too...
Don't feel guilty!!!