That’s how it’s done in the TF fandom? No wonder my fics never caught on.

Heh. XD Actually I’ve probably seen at least a dozen ways to do it, but it’s a common idea.  My detailed standard headcanon on how mechpreg works under the cut if anyone’s curious. 

Firstly, (almost) anyone is capable of carrying, and anyone is capable of siring.  Their frames are made of living metal around a sort of biomechanical “protoform,” which is the most vulnerable part of them.  It’s melded to the living metal, and the presence of a newspark can trigger (almost) any frame to begin the carrying process.  

The first stage in carrying is a spark merge to create a newspark.  A spark overload creates an excess of spark energy to flow outward, and when kindling (conceiving), that energy mixes and settles into the chamber of the mecha who will carry.  The mecha who overloads first will be the one to carry in this situation, and to create, the contributing party(ies) should hold out as long as possible.  My thinking on it is that the first mecha who overloads suddenly has a lower “charge” in their spark, and then when the other(s) overload, the energy is sucked into the first chamber to fill the sort of energy sink left there, and balance out the charge between both frames.

If enough excess spark energy is spilled into the first overloading mecha’s spark chamber, a newspark can form, and will orbit the carrier’s spark, taking energy slowly from them to grow.  The sire(s) may continue to merge with the carrier during this stage, and contribute more energy as well.  

It is also my thinking that one way of inducing an abortion of a newspark, especially in the early stages, is to merge with someone who was not part of the creation process.  The frequencies in the sparks would be too different, and could cause the newspark to lose its orbit and disperse.  There is no real consciousness at this point.  

There can be as many sires as can logistically be included in the initial spark merge.  Three is easy.  Ten would not be.  Four to five?  I can see it, if we had some creativity and flexibility going on.  Obviously you’re not going to get fifty sires in on an initial merge.

After the newspark is created, anyone can become a sire to it.  The sires involved in the spark merging process are the spark sires, and everyone else is a frame sire.  That number is literally only limited by time.  That number can also be zero.  Carrier’s can build a new frame solely from material taken from their own.  The creation might not be as big, but provided the carrier is healthy and has good nutrition (mineral and metal supplements in their diet to replace in their frame what the new one is taking), yeah, they can easily do it alone.  

Frame sires contribute through transfluid, which is collected by a carrier’s frame and the matter in it repurposed and used for creating the new frame.  Depending on the ‘verse, sires sometimes need to take supplements, or sometimes there’s coding that kicks in when there is a carrier around, or when a mate is carrying, or when they smell a carrier… there’s a ton of different options.  But there could literally be thousands of frame sires, or one, or two.  It’s a choice made by the carrier and anyone they choose to involve in it.  

This of course can lead to a lot of fun kinky happenings in which there’s an almost primal quality given to the mating and transfluid, which for some reasons squicks me a bit in humans (something about organic pregnancy has just always bothered me … there’s this visceral No reaction to it), but that I find endlessly fascinating and awesome when done with robots.  *shrugs*

Once the new frame is strong enough, the newspark will travel to it and gain sentience for the duration of the carry, until it is ready to separate from the carrier.  Separation is usually painless and easy, if a bit uncomfortable and unsettling for new carriers especially, and involves the front of the armor splitting open to allow the little one to detach from the supportive systems and climb out.  And voila, babby robot. 

And of course, depending on the nature of a prompt or a bunny, little things can be tweaked here and there.  I wrote in an egg ‘verse that I came to really, really love, and you can switch it so there are sires types and carrier types and non-creating types, make it so that only kindled mecha can carry, have a ‘verse where only the Prime can spark sire, etc. 

But, yep, that is how I do creating in the world of Cybertronians.