Would we survive being unbirthed?

[Groblek]

Well put, Dreamweevil. Also, speaking as a bio geek, reversing the birth process enough to allow an umbilical connection to your new mother and for her to become pregnant with you is trivially easy if you're compairing it to what it would take to shrink someone down that small with their mind intact in the first place.

Also, welcome back! It's been a while since we've seen you around here.

[dreamweevil]

Groblek: Thanks! Yes, it's been a while-- too much to do these days! I see you've been very active here-- I'll have to catch up on some of your work!

[symbiote01]

In a word, no.

From the unbirthee's perspective, it would be extremely cramped quarters, with bands of muscle stretched dangerously around a sac that would also be stretched to the limits. Bands of muscle would separate exposing weak spots in the uterine itself. Adult bodies are simply not built the same as a fetus's- their heads are (relatively) smaller and their limbs bigger. If you could somehow get inside, your elbows, heels, and shoulders would probably be the death of your host when you accidentally twitched and burst through one of those weak spots in her uterine wall.

Even if you were to somehow able to physically get into an adult uterus without it rupturing, we would need a means to develop another amniotic sac, umbilical cord (a complex organ), and a placenta large enough to sustain an adult (which would have to be a lot larger than most people realize). All of these things are genetically part of the unbirthee, not the unbirther.

Here's a snippit from Wikipedia about the umbilical cord:

"The umbilical cord enters the fetus via the abdomen, at the point which (after separation) will become the umbilicus (or navel). Within the fetus, the umbilical vein continues towards the transverse fissure of the liver, where it splits into two. One of these branches joins with the hepatic portal vein (connecting to its left branch), which carries blood into the liver. The second branch (known as the ductus venosus) bypasses the liver and flows into the inferior vena cava, which carries blood towards the heart. The two umbilical arteries branch from the internal iliac arteries, and pass on either side of the urinary bladder into the umbilical cord, completing the circuit back to the placenta."

When a child is born, the umbilical vein and ductus venosus close up, and degenerate into fibrous remnants known as the round ligament of the liver and the ligamentum venosum respectively. Part of each umbilical artery closes up (degenerating into what are known as the medial umbilical ligaments), while the remaining sections are retained as part of the circulatory system.

So, those things would have to be regenerated or replaced (including, most likely, the liver).The prenatal heart has a similar but different function than post-natal- so it would need to be stopped and rewired to do so again. The lungs and especially the diapraghm would have to be 'un-developed' to shrink alveoli, prevent breathing impulses, and turn off the physiologically mature systems. In many cases, these are one-time 'switches' that fundamentally change the purpose, dimension, and in some cases the chemical make-up of parts of the body.

The parasitic placenta requires oxygen and energy from the host- to sustain an adult-unbirthee. The placenta is about 1/5 the weight of a newborn at birth- that requirement would be pretty large; even considering the minimal movement and altered oxygen requirements as explained above. So start growing yours now... if you weighed in at 180lbs, you'd need a placenta that weighed 36lbs. If you consider that a newborn only weighs about 7lbs, your placenta alone is more than five babies big...

*EDIT- Also, that placenta size is at birth- a time when it is starting to become inadequate for the job. So you should probably plump yours up a bit more if you are planning an extended vacation...

[Groblek]

Symbiote - to be fair, the OP was postulating a shrunken human to begin with, but yeah, that's a pretty good summation of why I rarely try to be "realistic" when writing a same-size UB piece. I'll be as realistic as I can, but the flat reality of the situation is that it's an impossibility without magic or similar handwavium. You didn't even get into the little fact that as it is, a mother's system is pretty close to overtaxed with two or three babies in a normal pregnancy as far as little things like her kidneys filtering out all the toxins are concerned. Managing it for an adult? Really not going to work.