If she's a friend that you would ordinarily be talking with this week, I'd ask her about the conversation. As you're feeling, it's awkward keeping secrets. Because she was talking loudly outside I would guess that she's not keeping it a secret.
I would consider that she, herself, may not be pregnant. She may be talking about an adoption or even another friend being pregnant.
If you'd ordinarily see her, I'd ask her about the overheard conversation. Start with a general statement such as you couldn't help but over hear her conversation about pregnancy. Then let her go from there. If she shuts down you know she doesn't want to talk about it. But I'm guessing she will want to share the good news.