Can forgetting or not taking prenatal vitamins cause a miscarriage?

Probably not: Very unlikely that forgetting your prenatal vitamins can lead to any serious negative outcomes. We prescribe them simply because it is so common not to get the appropriate amount in one's diet when pregnant and responsible for the developing fetus. Worry not but try to take them, especially Folic Acid before and early in pregnancy.

Possibly indirectly: Not getting adequate folate (folic acid) can lead to neural tube defects (spina bifida, etc) which may be sever enough for the body to abort the damaged fetus. Thus, it's important to take prenatal vitamins even before you become pregnant to prevent such tragedies.