The couple, who got married in August, are set to welcome a baby into the world next year, according to E! News.
The 30-year-old singer already has six-year-old son Bronx with ex-husband Pete Wentz.
The 'Hunger Games' star Evan, 26, recently admitted he wants to have as many children as possible with the blonde beauty.
Speaking last month, he said: ''I would love to [have kids]. As many as possible.''
Asked about married life, he added: ''It's the best thing ever.
''I've been with her now for a while so it just feels right. I'm happy to say she's my wife.''
Ashlee was said to be equally keen to give a sibling to Bronx, and sources close to the couple - who married in a romantic ceremony held at Evan's mother Diana Ross' lavish home in Greenwich, Connecticut - previously said they ''can't wait'' to see their families grow.
The insider said: ''They can't wait [to have a child together].''
Bronx already has some experience of being a big brother, as Pete also has three-month-old son, Saint Lazslo Wentz, with girlfriend Meagan Camper.
The Fall Out Boy rocker split from Ashlee in 2011 after three years of marriage.
Kai Whiston is back already with his second album of his own 'No World As Good As Mine', and this time he has the help of organic instruments.
Her debut album The Witching Hour is out soon.
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