Toni Harper met Sean Horton when they were aged 12 and 15 and he was a blonde girl called Sian.
They became good friends and Sean 'came out' to Toni about his gender and sexuality.
Sean, now 25, started hormone treatment and the pair fell in love as his voice lowered and he grew facial hair.
They're now going to get married and Toni, now 22, is due to give birth four weeks after their wedding thanks to a sperm donor.
Sean, from Knowle, Bristol, told the Bristol Post: "When I was younger I always wanted to wear boys' clothes. My mum just thought I was a Tomboy until I told her the truth.
"Now I just can't wait to be a dad. I can't wait to hear him call me 'daddy'."
Sean aged 12
The couple became friends while at school and grew so close that Toni was the first person Sean told that he wanted to be a man.
After leaving school, Sean and Toni drifted apart and Sean told his mum, Heather, who was very supportive.
The family went to their GP to find out about having a sex change and he was referred to London's Gender Identity Clinic.
Sean began hormone treatment when he was 20 and then and met up with Toni for the first time in two years.
Toni said: "He suddenly had facial hair and his voice had dropped down really low - I couldn't believe it was him.
"It felt weird at first, but we started seeing each other more and more until we started going out and growing closer and closer.
"When Sean first told me, I asked him how that could be possible. It was so difficult, but I knew what was going on deep inside."
Sean had a full mastectomy and then a year later he began genital surgery.
Sean said: "After the first surgery I was in shock. But now I couldn't be happier."
The couple had artificial insemination treatment with a donor sperm and Toni then became pregnant with their baby boy.
They then won a £12,000 wedding through the Bristol Post.
Sean said: "I believe this is fate falling on us. He wanted his first grandson as bad as we did, but when he was here we had three failed attempts.
"Then, after he passed away, we managed to get pregnant on the first attempt. We couldn't believe it. Toni said she will never forget my face when I found out."
Toni added: "It was the longest two weeks of our life after the insemination.
"We had tried before. It had been so hard and so disappointing. But we were so relieved in the end."
Congratulations Toni and Sean!