Why are babies often referred to as "HER"

Chuck1906

Since I am a new Dad, I have been doing a great deal of reading and researching things and one thing I can't seem to understand is why these writers of such articles including ones here on The Bump, refer to babies as "HER". Why can't they say Him/Her or He/She? I guess its just easier for them to say "her" for some reason. 

 For example here is something I came across that refers to the baby as "her". 

The American Academy of Pediatrics recommends that babies spend a few minutes on their stomachs two to three times each day. "Tummy time" will help strengthen muscles in her arms, shoulders, back, and neck. Plus, it helps her work on motor skills such as reaching, rolling over, and crawling.