Ashlee Simpson and Evan Ross want to keep their marriage ''fun''.

The 'Pieces of Me' hitmaker tied the knot with the 'Hunger Games' actor last year and Ashlee is keen on making sure the couple's marriage stays ''fun''.

A source told US' OK! magazine: ''Ashlee and Evan made a promise to each other that they want to keep their marriage fun and exciting.''

The pair are enjoying their last few moments of alone time together as they are soon to become proud parents of a little girl.

The star's sister Jessica Simpson recently held a ''chill'' baby shower for Ashlee at her home.

A guest shared: ''It was beautiful weather and her house was the perfect setting ... so gorgeous.

''It was very chill and almost like just a get-together than a traditional shower. It wasn't focused on games [and was] more like a big party.''

Ashlee's six-year-old son Bronx - who she has from her previous marriage to Fall Out Boy's Pete Wentz - is also looking forward to welcoming his little sister.

She gushed previously: ''I told him pretty early on [that I was pregnant] and he wanted to have a sister, so he's really excited! He's got a little brother. Now he's going to have a little sister!''