I did a modified natural FET in December. The theory is actually pretty cool.
With a natural FET, they will monitor for ovulation, and 5 or 6 days later, they'll transfer your embryo. In theory you don't need progesterone support, because after ovulation, your follicle will become the corpus luteum, which I believe is what produces progesterone. You may just want the clinic to check your progesterone level around transfer or a few days later to make sure all is good.
With a modified natural FET, like Misskika, I was on letrozole, which was to help speed up ovulation and it usually keeps estrogen down. I triggered with HCG and then was on HCG support in the second half of the cycle. Normally the corpus luteum will disappear during the luteal phase and then your period will start, but if you get pregnant, the implanted embryo will produce hcg that keeps the corpus luteum producing progesterone. In other words, our bodies will produce everything we need in a natural cycle (ideally). So in a modified natural FET, I took HCG support, to mimic pregnancy hcg, to keep my body producing progesterone naturally. I also took progesterone cream (Crinone) for additional support.
Long story short, if you ovulate fairly regularly, and don't seem to have issues with progesterone, a fully natural FET is probably good, especially if they check the progesterone.
If you do need a little extra support in the luteal phase, perhaps you could look into the hcg support. Caveat, you can't use pregnancy tests at home since you'll get false positives!
If you do need progesterone support, there are different kinds. They all seem to be effective, so it depends on what works best for you. Vaginal suppositories are the messiest usually. I've enjoyed the cream/ gel (Crinone) although I've only had to take it once a day, it might get messier if you need it more often. PIO injections can be sore but I find they provide the most support for me personally.
As for how each hormone affects you and your colitis symptoms, do you have access to a gastro specialist to find out whether progesterone or hcg will cause a flare up? My cousin has colitis and she was quite sick throughout her pregnancies, but I don't know if it was due to a flare up or just regular pregnancy sickies (like HG).
I'm sorry you have to deal with colitis on top of infertility. I hope that you are able to find some answers and a protocol that works for you!
PS Misskika, I'm so glad that you found a doctor that takes your issues seriously and I have my fingers crossed for you!
TTC since 2013
Unsuccessful IVFs 2014-2016
Unsuccessful known egg donor cycle 2018
Donor Egg Bank Assured Refund plan 2019
Two unsuccessful DE transfers 2019
Unsuccessful DE transfer 2020
DE transfer Nov 2020 - BFP
More about my journey in my profile