Yes, it is believed that bipolar disorder is hereditary, but there is no need to panic! Just keep a close eye on your child. One thing to bear in mind is that bipolar disorder is often mis-diagnosed as ADHD and ADD in school-age children. I remember, when I was little, that I used to go through whirlwind bouts of hyperactivity that used to puzzle my Mum!
The teenage years are crucial, as this is often the stage where symptoms first appear. I, for example, noticed depression really setting in about age 15 and psychosis a few years after that. Also, if your child endures a particularly traumatic event, watch for symptoms. As your child gets older, be open about what bipolar disorder is all about to foster a safe environment for him to talk about any depression/mania or other things that may crop up.
In the meantime, relax and enjoy your gorgeous new bub!
EDIT: FYI, bipolar disorder is strongly prevalent in my family.