During pregnancy, the body makes more hydrogen sulfide to help with making new blood vessels (pretty important when making little arms and legs) and to keep the uterus from contracting. This toxic gas breaks down quickly under normal circumstances, but if we're short of molybdenum (trace mineral, element #42, and used by the body in sulfite oxidase, the enyzme that breaks down sulfite), it can get stuck at sulfite, which is also toxic. If that sulfite builds up in the GI tract during the night, then throwing up would be the body's fastest way of getting rid of some of it in the morning. I took molybdenum for morning sickness, and it basically made the nausea part of morning sickness go away (although if I ate food with a lot of added sulfite, I felt ill anyway because the body can only make so much sulfite oxidase). I put all the supporting studies for this theory together and just published it with a medical hypotheses journal: www.sciencedirect.com/.../S0306987716300986