The "Dip It Low" hitmaker became an ambassador for the superstar's Savage x Fenty underwear brand in May (20), and earlier this month (Dec20), she announced her third pregnancy.
The baby will be her second with her singer partner Matt Pokora, and Milian reveals she was wearing Rihanna's designs when they got down and dirty.
Taking part in a viral Instagram challenge, in which users request people share a certain image from their camera roll, Milian responded to one fan who asked her to post a photo of when she was pregnant but didn't know she was expecting.
She replied with a posed picture of herself modelling a pink Savage x Fenty lingerie set, and explained, "not pregnant here yet...but...was the day the stork made a drop off...thanks @savagexfenty (sic)."
Milian had originally uploaded the sexy shot in August.
She and Pokora are already parents to son Isaiah, who turns one in January, while Milian shares 10-year-old daughter Violet with her ex-husband The-Dream.
Rumor has it, Christina Milian and Matt Pokora got married one day before they announced the upcoming arrival of their second child together. They allegedly tied the knot in a town hall in Paris.
Back in May, four months after giving birth to son Isaiah, she said fatherhood made her beau broody for more kids. "This is his first child and he's just in love. He's ready to make more," she claimed. "I'm like, 'Hold on, my body's not ready!' I just want to enjoy this moment!"
However, she confessed the idea of expanding the family quickly grew on her. "I'm like, 'Well, maybe, do we hurry up and keep going so that we can knock it out (complete their family)?' " so she said.