Dear Sister, I am sorry to hear this.... I still have not told my mum I am wearing hijab now and I am dreading it even though I am 38, so it seems ridiculous in my situation!
I would probably send flowers..... Eventually, she will learn that by becoming Muslim, your respect for your mother will be even so much more than ever before. We say that Paradise lies at the feet of the mother
Allah details in Qur'an how much He appreciates what pregnant women go through, I don't believe you will find such a thing in any other scriptures.
Wearing "hijab" was a command for Jewish and Christian women also but they did not keep it up. I hope that she will understand that by leading a healthy life (not drinking alcohol, not eating fatty port, taking care of these bodies that Allah gave us) that your life is improving, even if she cannot understand the spiritual aspect.
Good luck, I would send flowers and not bring up religion for a little while but I guess you have to be ready with respectful and positive things to say as the hijab will be a constant reminder every time she sees you. She will calm down, Insha Allah, and learn to love the decision you made, Insha Allah, Insha Allah. This is my prayer for you, sister