The impact of rate of nicotine metabolism, as measured by the nicotine metabolite ratio (NMR), on smoking behaviours during pregnancy: A scoping review

Smoking during pregnancy is detrimental to the foetus and the mother, but quitting smoking can be challenging for pregnant women. The nicotine metabolite ratio (NMR), the ratio between 3-hydroxycotinine and cotinine, indicates the rate of nicotine metabolism and is associated with smoking behaviours and cessation success in the general population. Our goal was to establish what is currently known about the rate of nicotine metabolism during pregnancy. A search was conducted from inception to January 2024 using PubMed, EMBASE, CINAHL, PsycINFO, Web of Science, and Medline. Inclusion criteria encompassed pregnant women actively using any tobacco or nicotine-containing products for whom cotinine and 3-hydroxycotinine levels were assessed in maternal saliva, blood, or urine samples. Exclusion criteria comprised non-English articles, passive exposure to smoking, and absence of cotinine and 3-hydroxycotinine measurements. In half of the studies, more than 75% of participants were White. Four studies demonstrated that NMR (i.e. rate of nicotine metabolism) increased during pregnancy. Two studies found opioid use can increase NMR, and higher NMR correlated with greater nicotine dependence during pregnancy. One study found that higher NMR was associated with heavy cigarette use during pregnancy. Similarly, one study revealed pregnant women with higher NMR are less likely to quit smoking. However, one study linked lower NMR to lower birth weight, particularly among African-American populations compared to White populations. NMR tends to increase during pregnancy, complicating smoking cessation. Although race is an important determinant of NMR, the limited range of ethnic backgrounds highlights a need for further research in diverse populations.