MA - here's the paper (lots of useful info)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3267524/
(I work in medical research, so it make sense to me - let me know if there's bits that don't make sense!)
Mostly they say natural (eg pessary PG is similar to injected P17) but this bit caught my eye. if they're both equally good at preventing PTL then why not just give vaginal??
Natural progesterone has documented properties of inhibiting uterine contractions [85,98,100], whereas 17-OHP-C seems to have no effect on uterine contractions [85,99]. In addition, natural progesterone has an established safety profile in the first trimester of pregnancy from more than 11 years of continued and ongoing use in infertility as daily progesterone supplementation and replacement in IVF cycles [101]. Furthermore, in a recent very large preterm birth prevention study of singleton pregnancies, no cases of miscarriage associated with the use of micronized natural progesterone were observed [102]. On the other hand, 17-OHP-C is associated with an increase in resorption (miscarriage) in pregnant rats [96], total embryo-lethality in pregnant rhesus monkeys [103], a signal for a 30% increase in miscarriage in a meta-analysis of 17-OHP-C clinical studies [81], as well as an imbalance in miscarriage associated with 17-OHP-C in the largest placebo controlled randomized trial published to date [83]. In a study by Rebarber et al. [104], patients who received prophylactic treatment with 17-OHP-C had a higher incidence of gestational diabetes (odds ratio 2.9 [95% CI: 2.1-4.1]) than those who were not treated. The latter study suggests that treatment with 17-OHP-C may be associated also with increased maternal morbidity that is an additional safety flag.