I work at a daycare, and because Fifth disease was going around, I had to disclose to my boss that I am very newly pregnant (Fifth disease can be dangerous in the early stages of pregnancy). This happened a few weeks ago, and since then I have also disclosed a few personal details - in confidence - about my first ultrasound and some spotting to my boss. I returned from a day off of work to find out that she has told a few of my co-workers not only that I am pregnant, but some of those personal details. She also very often asks me about it in front of people who don't know, making it very hard for me to conceal the information. I am feeling very betrayed and like I can't trust my boss. Is this breech of confidence something that can be disciplined if I bring it to HR or head office? Am I overreacting?
You are not overreacting.
This is unacceptable, especially when you're in the early stages.
Pregnancy is generally considered personal information, and as a general principle, employers should not be sharing personal information without your authorization. You don't need to tell them you don't want the information shared. This is the default position.
In addition, miscarriage is not uncommon in the early stages of pregnancy, which is why people like to keep such things private. It can be difficult to explain that the situation has changed. Some people are not aware of how common such things are.
With regards to a solution, you have to figure out what outcome you are looking for here.
Your first step is to talk to the boss and tell them how they should change their behaviour. In addition, you should stop sharing information with them that you don't need to. Don't give them information about ultrasounds etc. Keep it to what they need to know.
It's OK to feel betrayed, but they probably didn't go out of their way to betray you. They probably had no idea that their behaviour is unacceptable. We would like to think that people in leadership positions would know their obligations, but this is not always the case.
For the sake of this answer, I'll assume you made it clear you wanted the information kept confidential and your boss is knowingly spreading it without your approval or consent.
You're certainly not overreacting. Anything you tell your boss in confidence should be kept confidential unless there's a serious, business-related reason to disclose it. Her behavior was highly unprofessional and you have every right to be upset. As far as seeking disciplinary action against her, it really depends on your company's policy and culture. I believe it's unlikely that HR would punish her in any meaningful way. The most you could hope to gain would be to get her to stop spreading this information without your permission but at this point, it sounds like it's already out in the open and it may not be possible to get the cat back in the bag.
I could certainly understand why you may not be able to continue working with this person and I think your best bet would be to find another job or request to transfer to a different manager/location.
It could be that your boss misunderstood the interaction. Maybe she thought you guys are friends and she wanted to spread the news. Thus far, it doesn't sound like she released the information to harm you.
Nex time, I think you should be clear. "Boss, I am in the early stages of pregnancy and I haven't told anyone in the office yet. I appreciate it if we can keep things between us for now. I'm only telling you this because of the disease that may affect me personally."
I do think you should call your boss to another meeting and this time go over you hope she can keep the news private and hope it doesn't spread.