I would follow the 3 month guideline... aka.. only tell people after 3 months.
You'll have the next 8 months to be working, if you start right away, and if it hadn't taken several months to apply for this position you'd already be working.
Will they be annoyed? Probably. They'd also be annoyed if you got in a car accident and were in hospital for 2 months, and had to deal with surgeries and pain meds. Ditto if 6 months in you got headhunted. You can't live your life not doing anything that might annoy another person or company. Our personal lives are our own durn business. Would you never get in a car, because of the likelyhood that it would annoy your company if you got injured? Would you turn down a better job, because you'd only just come on board with this company? You're an adult woman, who they have to assume is having sex, and who may be planning on having (or already have) a family. It's your personal life, and they're not allowed to ask you about it in the interview process (by law, although we frequently volunteer personal info), because no company is one's lord and master. It's your life. LIVE it as you choose to.
I like Hepburn's Mom's advice "You can't make eveyone happy no matter what you do, so always make sure that YOU'RE happy. At least that way one person is."
Now, if you're interviewing with Doctor's without Borders, or a Security (mercenary) Company, then it might be wise to beg out... but otherwise... do what's best for YOU. Trust me, any company is more than willing and able to look after themselves.