There are rare cases where there is a clear cause (e.g. a trauma to a pregnant woman), but many we don't understand. Pregnant women are a hyper-sensititive population at risk from all sorts of environmental exposures and conditions that wouldn't be considered a serious health risk to others. Neonatal environments have an influence rivaled only by genetics in their impacts on a child's well being, but few of pregnant women manage to do everything right, and our knowledge of what is best has changed frequently over the years.
Public health authorities have also not devoted much research effort in the past to distinguishing between conduct for pregnant women that is likely to be beneficial, that could cause birth defects or congenital disabilities (which may or may not be at a clinical level), or that cause miscarriage. For example, while it is widely known that drinking alcohol while pregnant can lead to fetal alcohol syndrome which has many negative health impacts on a child, it isn't as clear how often and to what extent alcohol must be consumed for this to lead to a miscarriage.
The lack of a good understanding about what causes miscarriage in many cases casts doubt on the wisdom of a Utah bill passed this past February.
Utah’s House of Representatives and Senate have approved a bill which would criminalize miscarriage caused by an “intentional, knowing, or reckless” act, such as drinking while pregnant. The bill is in response to a 17-year-old Utah girl who, while seven months pregnant, paid a man $150 to beat her up and induce a miscarriage. The girl was not successful and gave the child up for adoption. The Utah bill, which would not criminalize abortions obtained legally, is presently awaiting the governor’s signature.
Since abortion itself is a misarriage caused by an intentional act, the law also leaves legal abortion as an act that is legal solely as a specific exception to a general legal rule.
The Governor of Utah vetoed the bill on March 8, 2010 (Utah HB 12 bill text). To my knowledge, there has been no successful effort to override that veto.