So, I was digging around online, trying to find information on the progesterone that my doctor mentioned at my last visit. Apparently, it is vastly different from the progesterone that is used in early pregnancy to prevent miscarriage due to low progesterone. And there are a LOT of studies being done on this drug to see how effective it is, if it might be useful during first pregnancies, etc. Here's what I know now.
Alpha-hydroxy- progesterone caproate (17P for short), can help prevent cervical changes (specifically cervical shortening, also known as effacing, which is typically the step before dilation) when given weekly starting between 16 and 20 weeks gestation. The drug is given through an injection (although nobody mentions where... hip, arm, leg, what?). P17 also helps to relax the uterine muscles, which is also a factor in preterm labor. There was a study done by the NIH where women with previous preterm births got weekly injections and the risk was decreased by 35% (not bad!). The most recent studies, however, seem to indicate that the decrease isn't seen with multiples, only singletons.
If you want to find this stuff online, it's apparently marketed under the brand name Gestiva (I had a little more luck with that than with the chemical name).
So what this tells me is if I can manage a singleton pregnancy, we have a very good chance of carrying the baby to term (or really, really close) without a cerclage. If we get twins again, it's probably going to require a cerclage.