Heartbroken, scared, and sad...😔💔

Hi gals! My SO & I have been together for 9 years. We had a bump along the way but have gotten back together since and have been happier than ever. We are TTC and would like to get married. I'll be 38 next month and he is 34. I already have three children (16, 13, and 10) and this would be his first. His parents & sister know & are almost as excited as us & looking forward to having another baby in the family. But I am a little sad because I'm afraid that my mom won't be happy for me when it happens. She loves him & wants us to get married too. But she's made comments about getting my tubes tied before an "accident" happens. She's said that I'm too old & it's too risky for me to get pregnant again (my last pregnancy was high risk & kept being put on bed rest all the time; he was breached & born 4 weeks early). So, I can understand her concerns. But sometimes I feel as if there's other reasons for her not wanting me to have another baby. I've tried brining up the subject but she gets so bothered & a little upset at the thought of me getting pregnant again. I am so heart broken! How will I tell her the news when it happens without her blowing up I me?? Any advice?