Transfem

There isn't a whole lot of research but there is some:

https://transfemscience.org/articles/ (scroll down to prog)

I have decided against taking prog for now. Have been on E for ~18months.

You're getting a lot of different answers here.

There's no guidelines or good research on this. Doctors recommend waiting anything from 6 months to a year and a half. Sometimes it's to wait until a later tanner stage.

Ignore everything else you hear. There is no more useful information. The recent study showing prog might help with breast size started after 6 months. Everything else is guesswork or anecdotal, and our anecdotal data is very much lacking too.

I would opt against it just because people have a variety of experiences with prog in terms of mood, so starting them separately makes it easier to tease apart effects imo.

I would say no. The recommendations I've seen have all been for progesterone being added later, after one year of HRT at minimum. Its worth considering, but not right now.

During normal cis female puberty, progesterone comes in about 2 years after estrogen. Do with that information what you will. I'm pretty sure that mechanism evolved to make sure there was enough boob to feed a baby by the start of the menstrual cycle, rather than to arrest breast growth. Breasts will generally grow after that point, and progesterone is important for their maturation.

hmm, I personally would wait a year (I ended up starting around the 8 month mark), but I would take that advice with a grain of salt given there are people who start prog from day 1 and have excellent breast growth - at the very least it's not true that prog from day 1 will absolutely prevent normal or sufficient breast growth.

That said, it's sorta hard to know what all is in play because people don't always say what dose or route of administration they are taking. For example, taking prog orally does not sufficiently increase progesterone blood levels, and the liver basically absorbs >90% of the dose. So, would taking it rectally from day 1 cause the problems with breast growth people talk about, but taking it orally doesn't due to the liver filtering it?

For those reasons, I would be cautious and wait a year, and then take it rectally. (Just knowing this is an area where there isn't a lot of clarity, so that advice is liable to change as more evidence emerges.)

One reason to take prog, though, is that it does act as an anti-androgen, and that motivated me to start it a little early at the 8 month mark, to help me as I entered the final months before my orchi (which for insurance requirement reasons was only authorized after 12 months of supervised HRT).

Also, when doing monotherapy E injections, my E levels when high made it harder for me to sleep as long, so prog enabled me to sleep longer when I had those higher E doses.

absolutely not. prog only after breast development has finished. https://groups.io/g/MTFHRT/wiki/29602#Myths-and-what-not-to-do

Go ahead and start it now. There's no evidence that it interferes with breast development. In fact, the only study on it shows that it increases breast development.