Okay - so have basically read myself to death!
Results summary (as far as important for this group):
Progesterone supplementation is necessary and recommended by various sources in early pregnancy via ART - self AND donor cycles (but generally only deemed necessary until 8-9 weeks gestational age).... Why? Well the corpus luteum (the bit of the follicle that is left behind when the egg pops out of it) is responsible for the production of progesterone in early pregnancy. Frequently in ART cycles, either the CL is removed during egg collection OR the medications we are given prevent the necessary rise in progesterone to sufficiently support implantation and early pregnancy. If we don't supplement, implantation is unlikely or an early miscarriage will likely result.
BUT
Between the 7th - 9th week (depending on which source you read), something called the "luteal-placental shift" occurs - basically where the production of progesterone is taken over by the trophoblasts (cells of the placenta), and the CL doesn't mean jack anymore. Some even argue that once foetal heart beats can be verified, the level of progesterone being produced by the placenta is sufficient to cease supplementation. However, most clinics supplement at least until week 10 (this seems purely theoretical though, and no studies to date have demonstrated an optimum duration of progesterone support). It seems to make sense that most go until week 12 - as this marks the end of the first trimester and the risk of miscarriage globally drops at this point also.
ALSO, specifically for you Crystal - there are important effects of progesterone on the endometrium in terms of immunomodulatory effects. Adequate progesterone levels cause down-regulation of NK cells and other inflammatory nasties in the uterus, which is why progesterone support seems doubly important in women with immune issues. I have NO idea (just guessing here) - but perhaps THIS is why your Doc says 16 weeks? (But I did note you also said he does that for singletons as well...?). Ask for the justification though, because there is no scientific evidence regarding this. (Maybe take in the following article, and ask him to explain why his clinic has picked 16 weeks, and what the evidence is to support that).
This article was a good read: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3659905/
It has some info on immune effects of progesterone, Crystal. The last bit is about optimum duration of progesterone supplementation, and the longest duration they mention is 10 weeks for embryo transfer and donor cycles. If you're having trouble sleeping, you can read it!
Hope that answers any burning questions. Certainly makes me feel MUCH calmer about stopping in week 12 (although not completely!
)