Some women have found that vitamin B6 (the active form of which is pyridoxal 5'phosphate, PLP, or P5P) helps lessen their nausea and vomiting of pregnancy (NVP). (See https://www.ncbi.nlm.nih.gov/pubmed/25052410). Why does it help them? I believe I may have found at least a partial answer that is predicted by my sulfite-molybdenum theory of "morning sickness."
The enzyme sulfite oxidase, which catalyzes the conversion of sulfite to sulfate, contains two important parts: 1) molybdenum, and 2) cytochrome b5 type heme (http://jcs.biologists.org/content/125/20/4876.long, http://pubs.acs.org/doi/abs/10.1021/ja00099a024)
Where does heme come from? We make it, generally, out of iron, although sometimes we get it directly from food. Heme is a form of iron that is in meat and is especially high in animal liver (https://www.healthlinkbc.ca/healthlinkbc-files/iron-foods).
While liver contains many great nutrients, it should not be overdone during pregnancy due to the high amount of fat-soluble Vitamin A in it. A friend found that some pills high in dried liver (and recommended by her naturopath friend) made her morning sickness go away during a pregnancy two years ago, but she didn't take the pills regularly due to concerns about taking high levels of supplements; when she didn't take the pills, she was "sick as a dog," but given the amount of Vitamin A that might have been in the pills, I think she was wise to limit her intake of them. Fat-soluble vitamins are more prone to build up in the body, and too much Vitamin A from animal sources is known to be harmful to a developing fetus. Defatted liver pills should theoretically be a safer source of heme during pregnancy than non-defatted liver. After all, the reason the government requires adding Vitamin A to our skim, 1%, and 2% milk is to make up for the Vitamin A loss that results from cream removal. However, I am still looking into just how much Vitamin A is left in liver after the fat is removed.
Non-animal source iron is turned into heme in our bodies during a multi-step process that has been well-researched. The first step in the biosynthesis of heme depends on the enzyme aminolevulinic acid synthase, which is one of the many enzymes in our bodies that requires PLP as a cofactor (http://web.mit.edu/5.03/www/notes/porphyria.pdf, https://www.ncbi.nlm.nih.gov/pubmed/18314007).
Therefore, if a pregnant woman is not eating a lot of heme but does get enough iron from other sources, PLP supplementation could enhance the conversion of that iron into heme, which would then be available for use in sulfite oxidase. This in turn can help keep sulfite from accumulating in the body--sulfite accumulation apparently chemically "traps" PLP (http://pediatrics.aappublications.org/content/130/6/e1716), too, which is undesirable due to the many enzymes that depend on PLP--and so alleviate nausea and vomiting induced by sulfite in the stomach and intestines. However, a woman who already gets enough heme in her diet or who is deficient in molybdenum would be unlikely to receive much NVP-related benefit from PLP (i.e., vitamin B6) supplementation.