OK, I'm going to be talking about ways that sex changes can/could happen in a story. I'm not saying that this is how you have to do things, but my goal is to look at the actual biology/genetics and try to keep things compatible with reality to the greatest extent possible.

This is what's called a "hard" science fiction approach. You take the way reality works (to the best of our knowledge) and make the minimum changes required to achieve your story's goals.

So, since the first thing you need to know when writing about anything is how things are in the real world. This prevents you from making errors like giving Switzerland a seacoast.

So, we shall start with what the differences are between the sexes, and what causes those differences.

First of all, while "male" and "female" are the norm, they are not the only possibilities. Do a web search on "intersex" and you'll start to get some idea of just how varied things can be.

So, the primary source of the difference between male and female is genetics. Usually the X & Y chromosomes..

XX is usually female. XY is usually male. And there are a host of variations: XXY, XO, XYY and others more complex yet.

But apparent sex depends on a lot of other genes on other chromosomes. As well as environmental factors during pregnancy. I'm only going to briefly cover a few interesting (for our purposes) variations. The Intersex Society of North America (ISNA) website has a lot of references if you want to look into things further. Or invent your own.

I assume I don't need to describe what typical male & female bodies are like. So let's jump to what the differences are and which bits on each are related to the bits on the other.

Early Development

Early in pregnancy, other than at the genetic level, there are no differences between male and female embryos. At some point, they start to differentiate.

Tissues become either a penis or clitoris. Others become ovaries or testicles. Those are pretty obvious. But the other equivalencies are a bit harder to spot if you are a layman.

I'm just a knowledgeable layman, but that should suffice for this.

Analogous structures

• penis/clitoris
• prostate/skene's gland?
• ???/uterus
• testicle/ovary
• scrotum/labia minora

Yes, there's actually a tiny structure on the prostate that is what would become the vagina & uterus on a female. I can't find the article I read, but it does exist.

For the most part we don't need a lot of detail here. But I'll note that the testicles/ovaries start well up in the abdomen and descend into position.

Now, by birth most of this stuff is pretty fixed until puberty. But a few of the interesting (to us) things that can go "wrong" involve the penis/clitoris.

Hormones and similar chemicals can cause the penis to be very small, or the clitoris to be very large. And there are actually a set of "standards" used to decide which it "should" be. (Yes, until recently, doctors would, often without telling the family, "correct" genitals that feel outside the standards. That's one of the things the ISNA has managed to change to some extent).

So you could have a girl whose "hypertrophied" clitoris was cut down to a "proper" size (and yes, that does tend to affect sensation. But these doctors were all about "the child must look 'normal'"). Or boys who were reassigned as female as an infant because they had "micro-penis".

I've seen a picture of a girl baby with a hypertrophied clitoris (from a medical site that is long gone). It looks *just* like a penis, except it's at the top of her vulva, and the urethra doesn't go through it.

Oh yes, for boy babies common defects include having the urethra not go all the way up the penis and the "meatus" (pee hole) be somewhere on the shaft or lower. Hypospadia, if you want details. Often goes along with incompletely fused scrotum (remember, the scrotum starts out as two flaps and they join along the raphe (that "seam" you can see if you look closely)

Oh yes, and girls can have "fused labia".

Genetic oddities can also affect things at this stage. Androgen Insensitivity Syndrome (AIS)gives us a baby that looks female unless you check internally. It's got two varieties. Partial (PAIS) and Complete (CAIS). For our purposes, the main difference is that with PAIS large doses of testosterone (T) may cause some masculinization.

There's also a condition, 5α-reductase (5AR) deficiency, that results in XY babies that look female. Mostly found in the Dominican Republic, and called guevedoces.

Big difference between AIS babies and guevedoces is that AIS babies have a vagina, guevedoces don't. Once born things stay fairly as is until puberty. Then hormones start flowing and everything changes.

Puberty

Note that there are (expensive!!!) drugs that rather than blocking hormones, instead delay puberty by suppressing the signal that triggers it. Unlike hormone blockers these *won't* damage the gonads. So they are safe to use and once you quit taking them, your body will progress through puberty more or less normally. (So please use those instead of hormone blockers in stories involving pre-pubescent TS kids in modern (after 1990 or so) times). Best known is Lupron (leuporlide).

Both sexes get pubic and underarm hair. And there are some changes in sweat glands.

Boys get deeper voices because T causes their larynx to grow. This also creates the Adams Apple. And contrary to many stories, this is *not* reversible naturally. Once it grows, it stays that way.

Body hair and beard appear on males. Ranging from really sparse to quite heavy. This can be somewhat reversed by T blockers and estrogen (E). But not completely.

The penis enlarges. And the testicles probably finished descending sometime before puberty. In some cases long before. They get significantly larger too.

Also, at some point boys gain the ability to ejaculate (and loose the ability for multiple orgasms. I speak from firsthand experience :-)

Finally some bones get heavier/larger and you get a fair bit of extra muscle growth, as well as changes in how the muscles react to exercise.

For girls, the big changes are breast growth, periods, pelvic growth and some fat redistribution.

Pelvic growth isn't going to happen after puberty even if you go on E. And it certainly won't reverse if you take T. Ditto for getting the heavier bones found in males.

The AIS kids will undergo some pelvic widening, but not a lot, as they don't have enough estrogen. They will grow breasts, but small than XX females. And, of course, they don't get periods, In fact, that's usually when AIS is first diagnosed

The resulting body type is much like Jamie Lee Curtis (some suspect she's CAIS, but nobody but her doctor knows if she is or isn't).

The guevedoces (literally "penis at 12") are the real surprise. When their bodies start flooding their system with T at puberty, they start going male. bones, hair growth and significant growth of the penis. And the descended testicles growing makes it obvious that what looked like labia is a scrotum.

We've now covered the relevant natural processes. At least the common or "interesting" ones.

Hormonal effects

By supplying or removing hormones, you can induce some changes at any age.

Adding T after puberty will cause an increased sex drive in females. And male hair growth patterns (including male pattern baldness eventually. Yes, that's actually a real cased of "testosterone poisoning"). It also cause larynx growth resulting in the cracking voice and then permanently deeper voice. And there is usually significant clitoral growth. Nowhere near a "normal" penis, but still significant.

It also tends to stop periods. But not always.

Removing T from men causes some muscle mass reduction, lessens sex drive, and causes fat redistribution. Partly from lowered metabolic rate and partly because of the E that even the male body produces. Hair will get finer, but not go away.

Supplying E, especially if T is blocked results in breast growth, lessened sex drive, and finer hair. The testicles will tend to atrophy just from the E, and may suffer other effects that dictate their removal.

Contrary to popular belief, neither removing T nor adding E causes shrinkage of the penis. Instead, it shrinks literally from lack of use. Even if you don't use it, spontaneous erections in your sleep will maintain the size. But removing T or adding enough E removes those nocturnal erections. Which means the skin and erectile tissues aren't getting stretched. So they shrink.

Other things (like sleep apnea, or side effects of some drugs) cause the same lack of erections. And shrinkage. Over a few years you can go from 6+ inches to 2 inches. And if you are lucky, regular "exercise" (masturbation if that still works, or things like penis pumps) can recover much of the loss. But that, too, will take years.

Note this as a side effect of some sorts of chastity belts too if you have those in stories. Mainly the sort that are fitted so as to prevent or punish erections.

Unnatural transformations

Okay, we've covered natural changes, and some "unnatural" ones. So now it's time to consider how our desired "unnatural" changes would work.

Many of the changes we'd like can be "explained" by reactivating whatever processes are active during puberty. So we'll cover those first.

Breast growth is pretty much not a problem. Getting large amounts of growth after puberty finishes is not normal, but it's something that doesn't require huge amounts of hand-waving.

Getting it in less than months does require something extraordinary, but I'd say you could get away with A cups in week or so without trouble. B cups, maybe. Anything larger requires really speeded up metabolism or lots more time (or magic, which is outside the realm of this essay).

Male hair patterns/growth is not even a reach unless you want the character to get really hairy, really fast. Ditto for larynx change from female to male.

Growing a clitoris to the size and shape of a penis is a reach, but not biologically "impossible". Especially if we play the "reactivate puberty processes" card. Still a reach if you want even normal size. And the urethra will stay where it is. But you will have a foreskin.

Bone growth (pelvic widening for MtF, hand, feet, shoulders, etc for FtM) is a reach even with the "puberty" bit.

And reversing growth is going to require some very non-standard biological processes. Shrinkage of the penis from disuses is about the only thing that comes close and it doesn't fit well, and takes months/years.

Even so, while they aren't "real" processes, we can envision ways to do the shrinkages without "magic". They may be unknown biological process, but they aren't impossible. Just very unlikely. And if they proceed at any great rate, they'll require a lot of extra energy (so the character should lose weight or eat like a pig).

So lets look at some changes.

MtF

Breast growth is natural, unless accelerated. Any size that occurs naturally is possible given time or high metabolic rate & appropriate nutrients. Don't forget the order of development. nipples/aureoles enlarge and get more sensitive,. "Lumps" form behind them (and hurt like hell if bumped!). Growth tends to be "conical, then round out.

Tanner Scale Note that this is useful for other changes besides breast growth.

Hair loss weird, but doable.

Genital transformation.

Scrotum divides becoming two separate sacs (some species actually have bifurcated scrotums as a not uncommon mutation. Sheep for one). Testicles could stay there (ouch) or migrate back into the abdomen. converting to ovaries can be hand-waved.

Penis shrinks, urethra migrates. Note that if penis was circumcised, clitoris will not have a hood. Erectile tissues below the base of the shaft migrate somewhat to become the erectile tissues of the internal portion of the clitoris (which lie on either side if the vagina). Sensitivity goes way up.

Vaginal channel forms between the labia (former halves of the bifurcated scrotum). It goes under the prostate which become Skene's gland (or the G-spot). The vaginal channel and uterus form from this tiny structures on the underside if the prostate.

Skeletal changes get hand-waved some. They really should be slow, and accompanied by "growing pains" much like many of us underwent during puberty. If accelerated, the pains should be stronger and more frequent.

FtM

Breast shrinkage is weird, but not impossible. Shrinkage (and loss of sensitivity) of nipples/aurelae problematic, but can be hand-waved. Rate is a problem. Remember that more than 5/lbs week is unusually fast weight loss.

Clitoral enlargement, not a problem, migration of urethra odd, but handwaveable.

Labia fuse, uterus/vagina shrink/close. More hand-waving Possibly a heavy period to get rid of the excess tissues? If so, have heavy period to remove uterus. Ovaries transform to testicles while descending into labia minora. Vagina loses depth after period ends. Labia fuse. Ovaries finish descending into new scrotum

skeletal changes problematic.

Now we get into odd stuff

M to neuter.

Testicles shrink and retract into abdomen. If medical imagery done, they just get "reabsorbed" by body after going internal.

Scrotum shrinks and eventually winds up as smooth skin, likely retains "seam" (raphe). This actually happens in some cases of castration.

Penis shrinks and goes away leaving meatus just above former scrotum.

Prostate gets reabsorbed

F to neuter

Breasts, uterus & vagina proceed as in FtM.

Labia fuse, then shrink leaving urethra exit just above them.

Ovaries reabsorbed

clitoris shrinks & reabsorbed

Lets get weird

Note that "hermaphrodite" is a very much deprecated these days. That's because of all the baggage it drags with it. Instead, "intersex" is used as a generic term, and then all of the hundreds of intersex conditions are dealt with under their own technical name.

But "hermaphrodite" has exactly the sort of baggage I want for the following:

pseudo-hermaphrodites

Male

Type 1: breasts and penis (typical "shemale") no problem

Type 2: breasts, penis, vagina.

Type 2 requires the vaginal opening to go between the two sets of erectile chambers in the penis. Which means this will be very tight if the penis is erect. You've got a choice of an opening below/behind the scrotum, or having a bifurcated scrotum with the opening between the halves. Note that the testicles will get a bit of a beating during sex. especially with the bifurcated scrotum.

Female

Breasts, vagina, "penis".

For this, the penis is a greatly enlarged clitoris. No need to relocate the urethra. And you can easily hand-wave the clitoris having the appearance and size of a penis. Note that due to the way the erectile tissues go on the inside, the pseudo-penis will stick out or even a bit "down" rather than "up" the way "real" penises do.

Oh yes, semi-relevant note. Female hyenas have a clitoris that is the size if a penis and functional as one. Except the vaginal canal goes through it. Which means that they give birth through the penis. Ouch.

True hermaphrodite.

breasts, uterus, vagina, ovaries, testicles & scrotum. And a penis or clitoris.

Much like the female pseudo-hermaphrodite or type 2 male hermaphrodite above.

big problem is getting the hormones to work together without messing up the body, and avoiding self impregnation.

Options are where to put the urethra (female position: top of vagina, under clitoris/penis, or male position: running along penis/clitoris with opening at end) and whether to place the vaginal opening below/behind scrotum or between halves of a bifurcated scrotum.

How to transition from male/female to any of the pseudo or true hermaphrodite types should be pretty obvious. Just use the relevant parts of the changes from starting sex to the opposite sex.

And finally, we have the possibilities of changing cyclically between male and female or more complex cycles involving neuter or hermaphrodite changes.

Biggest problem with cyclers is that there's a the strong desire to have them on a monthly cycle, analogous to periods. That requires an awful lot of reshuffling in a not very long time (a week or less).

My advice is to minimize changes. especially skeletal structure. Keep the hips to a wide enough (if any of the stages are capable of bearing children) but no wider. Keep the breasts to a reasonable size, and consider keeping an A or even B cup while in male mode (Stu Rasmussen, the mayor of Silverton, Or, has a nice set of B cup breasts (implantsts no hormones) and hid them successfully for years while in male mode). Hair can be hand-waved a bit or just have any male stages not very hairy.