Ashlee Simpson has got baby fever following the birth of her niece.

The 34-year-old singer - who already has 10-year-old son Bronx with ex-husband Pete Wentz, and three-year-old daughter Jagger with current spouse Evan Ross - became an Aunt for the third time this week when her older sister Jessica gave birth to daughter Birdie, and has said the new bundle of joy has got her considering a third child of her own.

She said: ''I'm not pregnant, but it definitely gave me the fever a little bit.''

The 'Pieces of Me' hitmaker says baby Birdie is ''so cute'', and revealed Jessica's other two children - Maxwell, six, and Ace, five - are already doting on their little sister.

Speaking to Daily Pop, Ashlee added: ''Oh my gosh, she is so cute. Little Birdie. The kids were so cute - Jessica's kids with their little baby sister.''

Meanwhile, a source recently said Jessica was ''doing really well'' following Birdie's birth.

They said: ''Jessica is feeling good. The baby is doing really well. Birdie is perfect and Jessica is savouring every minute. Her family has been back to visit at the hospital and they are elated. They are just soaking it all in.''

And insiders recently said the 'Blonde Ambition' star's older children are ''so excited'' to be big siblings.

One source said: ''Maxwell was hoping to maintain her record of the biggest baby in the family [at 9 lbs., 13 oz.], and now Birdie has her beat by 1 lb. She's so in love with her little sister. Everyone is very happy.''

Whilst Jessica is happy to be a mother of three now, she previously revealed she had no idea how she'd managed to get pregnant again.

She said: ''I thought about [having another baby], but I didn't really know that it could actually happen. We definitely always love to practice. We actually weren't practicing very much that month so I don't understand. We try [to figure it out]. This might be a miracle baby.''