ADIPOSE

Adipose are a sentient bidpedal species that developed a space faring civilisation whose body is mainly composed of fat.

Their natural mean of reproduction remains unknown but when their breeding world became unfit for use in the early 21st century, they enlisted the service of alien agent to have their birthing matrix implanted in humans through ingestion. Each matrix was building an adipose baby from large fat deposits contained in their overwieght human hosts. The baby was then extracting itself from fat mass with minimal harm done to the host. Adipose babies are notorious for their precocious motor autonomy. As soon as they are born they are able to walk with an overall good sense of orientation and purpose.
Source: Doctor Who "Partners in Crime"