The 29-year-old singer shared the news on her Instagram, accompanied by professional photographs of her and partner footballer Andre Gray.
She wrote: “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.”
The couple met in 2016 and became engaged in May 2020, after Gray popped the question on their four-year anniversary.
Speaking on Bumble’s My Love Is… podcast, Pinnock revealed that the pair were watching a film at home when Gray proposed.
“It cut out and there was pictures of us, it was playing our song, then the screen changed to ‘Will you marry me?’ and he was on one knee,” she said.
The news comes ahead of a new BBC Three documentary set to air later this month featuring Pinnock exploring racism called Leigh-Anne: Race, Pop & Power.
“The majority of the film is me talking about my experiences, being the darkest member of my band in my very white pop world,” she told The Guardian.
“I really wanted people to see that just because I’m successful doesn’t mean I’m not going to be affected by racism.”
She added: “My reality is feeling anxious before fan events or signings because I always feel like I’m the least favoured. My reality is constantly feeling like I have to work 10 times harder and longer to mark my place in the group because my talent alone isn’t enough.”
Little Mix announced they would continue as a trio following the departure of bandmate Jesy Nelson in December 2020 after she revealed that being in the group was taking a toll on her mental health.
In an recent interview with Euphoria magazine, Perrie Edwards revealed that the three women attend group therapy together.
“We’re each other’s support system in a way because we’re sisters and feel every emotion together,” she said. “We can always lean on each other.”
Pinnock described it as “being around people that make you feel good and that bring something positive into your life”.