The Voice star is reportedly several months along after undergoing rounds of IVF treatment.
Although it will be her first child with her partner, Blake Shelton, she is already a mother to three children with her ex-husband Gavin Rossdale.
A source told Radar Online that Gwen's pregnancy had been months in the making.
'Gwen adores being a mum and Blake has had babies on the brain for months, especially after getting to know Gwen’s boys,' they revealed. 'They knew they didn’t have time to waste, so they planned this pregnancy out, down to the last detail.'
Gwen and Blake met on the set of The Voice when they were both judges on the series.
They have been together since 2015.