Infant & Maternal illness & death are complex with many factors. Clearly alcohol, drugs, STDs as a group are the major external factor in the US. Age is the major internal factor. Women at either extreme of the age range and their babies have many more problems than women in the prime of the child bearing age range.
Any legit study is going to compare alcoholic babies with alcoholic babies, heroin babies with heroin babies, and keep them separate from moms and babies with no external factors.
The role of the father is also very important. A woman who has a undependable, irresponsible sperm donor has internal chemical reactions... defenses.... that are translated to the baby... often what is translated is that the baby is not wanted, the baby is a nuiance on the career or social life of the mom.
The topic is complex,
In a day of antibiotics, Antiparasitics, and vitamins, there is no need for vaccines