From the Kallah Magazine blog:
The following appeared in a 5 Towns posting:Note that there are some who are able to differentiate between this practice and that forbidden in Tosefta Shabbos 7:1 or conclude that we rule against that source, although I'm not sure based on what. According to them, as R. Aharon Lopiansky wrote (link), "this still does not make this a commendable practice, but rather a tolerable one."
“I have a small ruby gemach. It is a segula for pregnant women to wear a ruby to prevent a miscarriage.. . Check with your local Rav for sources. If you would like to donate any ruby jewelry or money so we can expand the gemach you can call the same number. Thanks! Tzklu’ l’mitzvahs!”
...To give the poster credit, though, there is the suggestion to contact your Rav. I know of at least one rabbi in town who would tell you that such a thing is absurd. But perhaps it would work as far as the placebo effect does, which may have a calming effect, which is good to lower blood pressure. High blood pressure does endanger some pregnancies.