After the 12 week scan and will it come as a shock to them? If not, do it however, but I’d make it about them... they’re getting a new brother or sister, not worded as a mums having a new baby. They’ll probably be super excited!
If it’s going to be a shock, you’d like to think at 19, your DD will be mature enough to handle and support you
I had tell them by accident really. My friend who also didn't know said something and I didn't want my older children to think they were not first to know. They were all really wonderful. Job done now let's find something new to worry about.