The 29-year-old British singer announced her baby news on Instagram, posting a picture of herself wearing a green silk bra top and skirt with her baby bump exposed.
In the caption, she wrote to her 6.5 million followers, "We've dreamed about this moment for so long and we can't believe the dream is finally coming true... we can't wait to meet you."
Perrie Edwards was the first of her bandmates to react to the news, posting in the comments, "I can't stop staring at these pictures. You're a vision. Ily both so much! CONGRATULATIONS my beautiful sister!"
Leigh-Anne got engaged to soccer player Andre last May (20) after he proposed as they celebrated their anniversary in lockdown.
She recently praised her footballer partner as her "backbone" and admitted she finds his "pro-Black" attitude inspiring.
"Andre is like my backbone. If I didn't have someone like that through this experience, I don't know what I would have done," she told The Guardian newspaper. "We've always had amazing talks about (experiencing racism), from when we've met. I love how pro-Black he is. It's inspiring for me."