I'm pregnant for almost 4 months. Partly happy for having baby and partly stressing of job.. worrying!?

I guess it's time for me to inform my collegues n boss of my pregnancy as i have to go for a frequent check up(eg. once a month and getting frequent until i deliver).Now the problem is i'm really stress of how to let my boss n also the collegues know.Should I tell my collegues 1st before i inform my boss?I seldom see my big boss as it's a partnership company.All i meet everyday is the minor partners therefore the big boss might not even care of my appearance.I afraid that the minor partners is not as considerate as they are.They might fired me after knowing i am pregnant as i have to take up maternity leave in a few months time and have to absence for check up frequently.There is no rules in our company saying of how to handle a pregnant lady since i only started working for 5 months.This mean when i had maternity leave, i still not yet 1 year in this company.Should i wrote a letter to my big boss telling him i am pregnant n not to fired me as i need this job very much or otherwise?