Pregnancy is a biological process that has been very well studied by medicine. As a result, modern medicine helps a lot to prevent miscarriages and to prevent death of pregnant women.
While the latter clause is correct (maternal - and infant - mortality is reduced with medical care), the former is not.
Miscarriages have been a problem as far back as recorded history. The written record includes medicinal recipes to prevent miscarriages. The problem then, as now, is that it just can't be done very well.
From a 2000 Obstetrics journal article:
Human reproduction is extraordinarily wasteful. The reasons for this have taxed all of the contributors to this book. As we move into the 21st century it is sobering to reflect on the fact that we have failed to harness the power of the evolving revolution in molecular medical biology to answer the fundamental question: why is the fate of a fertilized egg so hazardous and so unsuccessful? The following account summarizes our limited knowledge of the epidemiology of miscarriage and then moves on to consider some of the medical causes of miscarriage...
Miscarriage is difficult to study, and the causes difficult to establish, so the literature showing that any interventions help (antibiotics, bed rest, supplements, etc.) is not plentiful. Smoking seems to have a deleterious effect on pregnancy. How this relates to early populations is a matter of conjecture.
Some good studies do exist, however. One good study in the New England Journal of Medicine followed an ideal group: women who had no history of infertility, who were trying to become pregnant.
Of 221 couples followed for 6 months, a total of 199 pregnancies were detected, of which 126 ended in live birth (therefore a 37% miscarriage rate). Because of the methodology, failure of blastocysts to implant could not be measured. The findings of this study were supported by others (31%), etc.: early loss increased when implantation occurred later than 10 days after ovulation. While the authors were hopeful ("There may be opportunities to increase fertility by extending the time during which implantation can occur."), they were also realistic ("Such interventions should be approached cautiously, however, because they may have unintended consequences with respect to the quality of surviving embryos.") Basically, you can't fool around too much with Mother Nature in this realm.
Has modern medicine been able to change this rate? Not really. Even with IVF, where thousands of dollars are spent trying to preserve a single pregnancy (certainly not the norm), miscarriage is common. The best way to increase the viability of IVF pregnancy is to select better oocytes, embryos, etc. In other words, maternal factors remain refractory to manipulation.
Even things obstetricians long took for gospel (for example, bedrest with pre-eclampsia) have little evidence of benefit to the fetus.
The literature is vast in this area (reproduction is an important field to study). I can write a book on how little can be manipulated on average (I will consider spending thousands of dollars on one pregnancy not average), but it wouldn't sell. What does sell are false claims, this one being as good as any claimed today:
To prevent miscarrying or if one that is apt to miscarrie goe a journey let her take this powder morning and evening whilst shee journeys: Take of Dragons Blood the weight of a silver two-pence, a dram of red corrall powdered, the weight of 2 barly corns of Ambergrease, Besar ye weight of 3 barly corns, mix all these together and keep them close stoped in little vial glasse when you use take as much of it as lie upon a penny in a little clary water at nighte when you goe to bed, and in ye morning fasting and sleep after it to use it till you are out of danger.