You all know the story of a women waking up to find herself heavily pregnant with some demon child or somesuch, or the pregnancy spanning a couple of days/weeks rather than the standard 9 months. Commonly referred to as express delivery (warning! tv tropes).

Think of the foetus as suddenly magically / pseudo-scientifically arriving, rather than being grown in the mother. Can it happen overnight as depicted in literature and film or does it have to take some time?

My question is related to Just how fast can the human body naturally adjust to such a sudden change in circumstances, without, um, exploding?

Acceptable answers should focus on

  • How fast the mother's body can physically adjust to carrying a 6-9 month foetus
    • I am thinking mainly on how fast the skin and womb can stretch without tearing
  • The mother survives the arrival ordeal
    • Dying of normal pregnancy complications is acceptable

Answers should not focus on

  • The method/reason why the foetus arrived
  • The actual growth/development of the foetus itself
  • The birthing labour itself
    • For my purposes the women is not expected to give birth to the arriving foetus.
  • The society technology level. I am asking how fast the body can adjust and survive as naturally as possible
    • Unless medical intervention of some sort is essential to survival (I'm thinking painkillers, lots and lots of painkillers)
    • In my story this Express Delivery occurs more than once in different time periods with different tech levels


  • I have handwaved the mother's body chemistry for now, by saying that the women is already pregnant. I haven't figured out if the original pregnancy is as far along as the arriving foetus. Probably not.
    • This needs more development and comments are welcome but not required
    • For now the original foetus should have some chance of surviving as well (not necessarily guaranteed)

Tag Note

Aside from the sudden appearance/disappearance of the foetus, everything else should be as realistic/normal as possible.

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    $\begingroup$ Surely, if you can implant a fetus into someone, you can make the rest of their body be able to handle it. $\endgroup$
    – Carl
    Commented Nov 11, 2017 at 22:04
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    $\begingroup$ @Carl, well, first I need to figure out how much the body can do itself before I decide how much magic/technobabble to spout to explain the rest. Do I need to prepare the mother's body before implantation or will she just stretch to accomodate the new arrival. I'm trying to avoid having to drug or give magical potions etc beforehand in order to allow the belly to swell/stretch to that size. The implantation isn't really the issue, it's how the mother's body reacts that I'm interested in. $\endgroup$ Commented Nov 11, 2017 at 22:22
  • $\begingroup$ Are you asking about a 'normal' pregnancy (from impregnation to term; a fetus follows normal development stages) progressing at extraordinary speed or implantation of a fetus that has already developed elsewhere? I think we need a clarification here to give you the best answer. $\endgroup$
    – Olga
    Commented Nov 12, 2017 at 1:42
  • $\begingroup$ Have you considered the fact that evolution pushes pregnancy to shorten already? Shorter pregnancy means less time when it's easy to get eaten and harder to hunt or graze. Why do you think that it is not already as short as realistically possible? $\endgroup$
    – Mołot
    Commented Nov 12, 2017 at 10:25
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    $\begingroup$ @Olga, I figured express pregnancy was a 2 part problem. (1) How fast can a foetus can grow from impregnation to term and (2) how fast can the mother adapt to the growing mass. Anything to do with the actual growth of the baby/energy expended by mother (1) would be limited by how fast the 'mother' can actually stretch (2). So for now, I'm mostly focussing on implantation/teleportation/magical arrival of a foetus that has already developed elsewhere etc. But I know this has issues. Once I have an upper limit for the mother stretching (2) I can work on a question about the foetus growth (1). $\endgroup$ Commented Nov 12, 2017 at 12:25

2 Answers 2


I don't think this is realistic at all - you'll have to call on magic to make it work. The human uterus is not like a balloon, which can just be inflated at will. It would go about as well as if you suddenly had a meal the size and weight of the baby and placenta teleport into your stomach!

Things I can think of are:

  1. The uterus is the 2nd toughest muscle in the human body. (The heart is the 1st). In a 'wrestling match' between the uterus and the suddenly arrived and quickly expanding foetus, the foetus would be squashed like a grape. Unless the 'demon foetus' has super-strength or something.
  2. The reason human women can bleed to death when pregnancies go wrong is that the human placenta (which is part of the foetus, not part of the mother) 'burrows' through the wall of the uterus, so that the placenta is directly bathed in the mother's blood. (Other species have layers of maternal tissue between the mother's bloodstream and the placenta. Ours is more efficient, theirs is safer for birth). If your 'demon baby' placenta does its burrowing faster than the uterus can grow to keep up, it will go straight through the uterus and start burrowing into other organs, such as the intestines, pancreas, spine or whatever. The technical medical name for this is 'fatal'!
  3. Where is the mass for this baby and its placenta coming from? If baby and placenta together weigh 4kg, that 4 kilos has to come from mum's body. She's going to be abruptly short of all sorts of vital minerals and nutrients, such as calcium for baby's bones, phosphate for its bones and nervous system, glucose to run its brain, and so on. Again, this will probably be fatal, but probably for the foetus rather than the mother, as she won't be able to provide the minerals and nutrients it needs fast enough, since she's limited to transporting them around her bloodstream as normal.

You might want to look at uterus growth rates for mothers carrying twins or triplets, to see how much faster that is that for singleton growth.

  • $\begingroup$ Some good points. Point #3 while normally valid for these scenarios, is actually outside of scope. I was trying to limit the broadness of the question and am only focussing on how fast the body can adapt to such a sudden mass arriving (not the actual growth of the baby). As I mentioned, I am having the baby arrive 'suddenly' and not grown inside the mother. Thankfully won't need to deplete her of 6-9 months worth of nutrients...just whatever nutrients the baby needs while it is kept inside her. Just need to figure out just how suddenly 'suddenly' really is. $\endgroup$ Commented Nov 11, 2017 at 22:30
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    $\begingroup$ @EveryBitHelps. Random thought... if there's no need to keep mum alive, and all it is after is a warm body to incubate in, then the demon baby could arrive in a man as easily as in a woman. Just like a parasitic wasp lays its eggs in a caterpillar or spider. Or a face-hugger lays its eggs in John Hurt! :-) $\endgroup$
    – DrBob
    Commented Nov 12, 2017 at 19:05
  • $\begingroup$ that is hilarious. So. How stretchy is dad? $\endgroup$ Commented Nov 12, 2017 at 23:48

The model is not a pregnancy. The model is a cancer. Cells that divide as fast as they can with no regard for the host, or each other, or the future.

Leukemia is one of the fastest growing cancer cells there is. How long would it take to produce a 22 kg demon child?

from Nat Methods. 2012 Sep; 9(9): 910–912. Direct observation of mammalian cell growth and size regulation Sungmin Son et al

Doubling time of malignant lymphocytes = 11 hours; assume 12 for easier math.

800,000 malignant lymphocytes / ml. 800,000,000 malignant lymphocytes / 1000 ml 8,000,000,000 in 10,000 ml or 10 l = 22 kg (assuming weight of water)

Assume doubling every 11 hours: 1 cell to 8,000,000,000 in 15 days.

Double checking from a different source:

Skipper HE, Perry S. Kinetics of normal and leukemic leukocyte populations and relevance to chemotherapy. Cancer Res. 1970 Jun;30(6):1883-97.

A single Ll 2 10 leukemia cell isolated with the aid of a micromanipulator often will (if successfully transplanted) give rise to l09 leukemic cells (the lethal number) about 15 days later (68, 71). The doubling time of the total population and the median cycle time of dividing L1210 leukemia cells are approximately the same until the number in the host approaches or exceeds 108 (68, 71). Also, the increase in cell number is essentially exponential over the range of 1 to approximately 100,000,000 cells; then population growth becomes asymptotic (68).

So about 2 weeks to grow a demon child made of leukemic blasts, using the maximum rate of actual cell growth in an animal host. Leukemia is bathed in nutrients and does not need to worry about establishing a blood supply - for a solid tumor that needs blood vessels it might take a little longer.

I thought I would check the observed doubling times of solid tumors. These need a blood supply like a demonchild, constitute a single lump of tissue. The fastest observed times for sarcoma mets and lung cancer are slower than leukemia; days instead of hours.

Mehrara E et al. Specific growth rate versus doubling time for quantitative characterization of tumor growth rate.Cancer Res. 2007 Apr 15;67(8):3970-5. cancer cells doubling times

But as regards the prospects of waking up with something huge growing in you - once there are a lot of cells those last few doublings mean things get big in a hurry.

  • $\begingroup$ But that is assuming an ideal limitless fits source. But as the number of cells increases the Axialis of foods decreases which limits the growth rate... $\endgroup$
    – Pliny
    Commented Nov 12, 2017 at 18:55

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