Loom

The Loom (or Looms) were a genetic weaving construction created by Rassilon following the Pythia's curse of sterility on Gallifrey. (NA: Cat's Cradle: Time's Crucible) They could literally weave a fully grown (though not matured) Gallifreyan. The Loom also kept tally of all the people it birthed, keeping track of how old each one was and how many regenerations each had gone though.

A Loom was set into each of the Houses of Gallifrey and each House had a specified number of Cousins which could exist in the Family at any given time. (for example, The House of Lungbarrow was alloted fourty-five Cousins) When a memeber of a Family died for the final time, the Loom would weave a new Cousin into the Family. Cases did exist when an additional cousin was woven (such as the Doctor's cousin Owis) but were extremely rare, as such a thing was illegal. (NA: Lungbarrow)

When he was only five years old, the Doctor boasted that he could remember existing in the Loom before he was actually born: "“I can remember waiting to be born…It was like being all strung out. All unravelled inside the Loom. I was spread really thin…I couldn’t think.  Not put thoughts together.  … But I knew where I was and what was happening. I couldn't wait to get out. And then I was born. My lungs nearly burst. The first rush of air was so cold....”" )  The truth of this claim is debatable as it's entirely possible the Doctor was simply trying to shock his tutor at the time.(NA: Lungbarrow