As you might imagine, there is a wealth of research on pregnancy and childbirth. However, it might surprise you to learn that, despite all of this information, healthcare providers still don't have definite answers to many of the questions surrounding childbearing.
What types of research exist?
The "gold standard" of clinical research is the large double-blind study, wherein one group of participants receives the treatment being studied and one group does not. These groups are assigned randomly in order to reduce bias. Neither the people participating in the study nor the people conducting it know who is actually getting the treatment and who is not.
This type of research is very hard to conduct among pregnant people. The health and wellbeing of the baby must be considered, along with the health and wellbeing of the parent. It is especially difficult to design a "blind" study when considering holistic alternatives. For example, if researchers were doing a study on the effects of hydrotherapy on childbirth, it would be obvious to the person in the study, their care provider, and the researchers if she were using a shower or bathtub during labor.
Care providers must depend on research that might not meet the double-blind gold standard, but that has credibility because it has been repeated several times with similar results, or because it tested the same participants before and after an intervention. They also rely on case studies, their own knowledge base of physiology and biochemistry, and on the practices of other obstetric providers.