Ashlee Simpson's Halloween baby girl

Ashlee Simpson and Pete Wentz are reportedly set to have a Halloween baby. 05/31/2008 - Ashlee Simpson Wentz, Pete Wentz - Palms Place Hotel and Spa Grand Opening - Arrivals - Palms Place Hotel and Spa - Las Vegas, NV, USA © PRN / PR Photos

05/31/2008 - Ashlee Simpson Wentz, Pete Wentz - Palms Place Hotel and Spa Grand Opening - Arrivals - Palms Place Hotel and Spa - Las Vegas, NV, USA © PRN / PR PhotosThe couple - who are expecting their first child together later this year - were said to be thrilled when they found out they were expecting a daughter due on October 31.A source told America's Star magazine: "Ashlee and Pete are on cloud nine. The doctor told them they actually conceived the baby just before Valentine's Day (14.02.08) while they were on vacation and has given them an initial due date of October 31."Fall Out Boy rocker Pete - who loves the spooky holiday - is reportedly especially pleased he will welcome his new baby into the world on the scary day.The source added: "When Pete heard he might have his baby on Halloween, he went nuts. For a rocker type like Pete, that would be just too perfect!"It was recently reported Ashlee, 23, has embarked on a strict exercise regime to avoid gaining too much weight during the pregnancy.A source said: "She's still clinging to the hope that she can limit her total weight gain to no more than 30 to 35lbs. So she is doing low-impact exercises, such as riding the stationary bike and Pilates."