because vs. since
*Many pregnant women run the risk of having a premature birth by leading an unhealthy life.
This seems to imply that the women want to run the risk. See by. Through is not possible for the same reason.
Other possibilities include:
There is probably a rule to account for 1-4 that can be extirpated from Quirk et al., but I can't put my finger on it at the moment. One could also discuss life vs. lives. The difference between since, as, and for, according to Swan, is that since is "a little more formal" than as (§83), but in AE as is less common than since, and, if anything, more formal. For is very formal, and "suggests that the reason is given as an afterthought; it never occurs sentence-initially.)
The question I am focussing on here involves 6-13, and especially 6-9. What 9, 11 and 13 have in common is that they are all adverbial clauses that Quirk et al. call "disjuncts," whereas because-clauses are "adjuncts." This means they behave differently syntactically. For example, you can say
It is because they lead an unhealthy life that they run this risk.
but not
*It is since they lead an unhealthy life that they run this risk.
(See Quirk et al. §15.20 for more such syntactic tests.) 8, 10 and 12 are unacceptable because since, as, and for in this sense (reason) can only be used as disjuncts. The comma represents a pause and falling intonation, which mark 9, 11, and 13 as disjuncts.
More to the point, the semantic difference between because- and since-clauses of reason is that the former "indicate a cause or reason so essential that they are integrated into the sentence as adjuncts, [whereas] nontemporal since-clauses have a looser relation, more resembling nonrestrictve relative clauses..." (Quirk et al. §8.132). Swan says "because is generally used when the reason is the most important part of the sentence" and is more likely to come at the end, whereas since and as are used "when the reason is already well known, or is less important than the rest of the sentence," and appear much more often at the beginning of (§83; cf. also Quirk et al. §15.47).
The situation is more complicated with because, because (!) it can be used in a way very similar to since (i.e., with a comma, as in 7), in which case Quirk et al. call it a "non-restrictive adjunct" (most adjuncts being restrictive, most disjuncts nonrestrictive), but it requires a rather contrived context to illustrate the difference. I think it is simpler to consider because, at least semantically, a disjunct, since 7, 9, 11 and 13 are very close to equivalent in meaning, except for the difference in register.