So, I went onto PubMed & found a few articles.
One in particular, in the American Journal of Epidemiology is titled, Self-reported Vitamin Supplementation in Early Pregnancy and Risk of Miscarriage (2009). I downloaded the PDF, if anyone would like me to email it. Basically, the study concludes that prenatal vitamin use (self-reported) was associated with a DECREASED risk of m/c. One fault I see right away is that 95% of the study participants reported using vitamins in early pregnancy. I'm not sure that a solid conclusion can be drawn from such a small percentage of non-users. Although the entire study was large (>4k participants), so the actual number of women is decent. Also, they cannot exclude other factors, such as the fact that vitamin use is presumably associated with other health-conscious behaviors, and (present company excluded) non-vitamin use may be associated with lack of concern for healthy habits. Also, they used self-report, so it is likely that vitamin use was over-reported. That's just what I got from reading the abstract. I have the full-text, though, if anyone would like it.
Just to clarify, lest I offend anyone: Of course, I'm talking about mainstream, general public here. Obviously, all the moms on this board are health-conscious, evidenced by the fact that we're on here talking about this, and I am certainly not implying otherwise. I do not judge anyone for ANY conscientiously-made decisions.
I also found one article linking prenatal folic acid supplementation with infant Bronchitis in the same Journal. And one other linking low prenatal folate and B6 levels (measured by plasma) to m/c. That one was looking at China, and was published in the Journal of Obstetrics & Gynecology (2002). I found one longitudinal study (4 years postnatal) of randomized DHA supplementation, which concluded that there is NO evidence that DHA supplementation increased any measurable factors (cognition, &etc.) (2014). Finally, I found one study suggesting that prenatal vitamin use (2nd trimester) may reduce risk of autistic behaviors at 4-5 years.
That was about all PubMed had to offer on the topic, except for a few 3rd-world studies, and I'm assuming those probably don't apply. I also found two studies suggesting NSAID use in early pregnancy is NOT linked to m/c, and another that modified that conclusion to say that race is a factor, and caucasians are more susceptible to NSAID effects.
When I have more time (haha), I'd like to look into the benefits of natural nutrients (that is, actual foods) versus supplements, and lab-created supplements vs. natural/organic ones. I wonder if there's any published data about it.
I can get most of the articles through UF, so let me know if you want to read any for yourself.