Have anyone got newly employed but suddenly found out you are pregnant. How do you tell the HR?

I just joined the company today and I also just told my boss about it even though I'm at week 9 (wanted to tell after 1st trimester) as my followup appointments with my doctor are all during weekdays - so I'll have no choice but to take unpaid leave and all. I figured it'll be easier to explain to my boss if I'm taking a couple of hours off or a full day off for any checks and scans. I haven't spoken with the HR - will be doing it tmr but hope they won't find an excuse to fire me!