How can I convince my dad and stepmom to get me a Guinea Pig with a baby on the way?

Hi. First of all, I REALLY want a GP for my birthday. I used to have a Bunny, but it died of a genetic disease. I took really good care of my bunny and am a huge help with my little sister and stepbrother and our dogs, so I am very responsible and mature. But my stepmom is pregnant and so my dad told me that I didn't need a GP, the baby would be my pet. I am planning on showing pictures and writing a letter, but I'm afraid it won't be enough. What should I put in my letter? and what are some other ideas to get them to say yes? PLEASE HELP!!!

-RIP Thumper :(
The GP would be in my room, so my stepmom wouldn't be exposed to it, and i would wait for the baby to be born, but i want it as a bday present and my bday is in 2 mos.