There are three main reasons adverse reactions happen. There’s the dose—obviously.  But there are several factors common to birth that increase the absorption rate of oils; this is a major reason why even experienced oil using mothers are more likely to have an adverse reaction while in labor. There’s also a wild card category that’s really important to know about: levels I & IV Allergic Contact Dermatitis (immediate & delayed hypersensitivity). Even if the dose and absorption are safe, the mother can still have a reaction between her immune system and the oil. You’ll learn what a reaction might look and feel like (it’s signs and symptoms), how long it will most likely manifest for, which types of reactions get better with time and which ones get worse. The different types of reactions also respond to different first aid measures—so all proper first aid training should start here: with the causes, types, and signs of adverse reaction.