The Big Bang Theory actor Johnny Galecki and girlfriend Alaina Meyer expecting their first child

Actor Johnny Galecki and his girlfriend

Alaina Meyer

are set to welcome their first child together.

The 44-year-old Big Bang Theory actor and Alaina, 21, made their relationship public in September last year.

"We are absolutely over the moon to announce that we will soon be welcoming a little one into this crazy and wonderful world," Galecki posted on Instagram.

"We ask that you please respect our privacy during this celebratory time for us and our families. There truly is love out there for all. We hope ours is the ember of yours, as we feel yours is the ember of ours," he added.