The two sets of analyses presented suggest that meconium can be used as a biological matrix to measure prenatal tobacco smoke exposure. Meconium tobacco smoke metabolites were positively associated with self-report and serum biomarkers of prenatal tobacco smoke exposure. We observed a dose-dependent relationship between the number of serum cotinine measurements consistent with secondhand or active tobacco smoke exposure during the latter two-thirds of pregnancy and meconium tobacco smoke metabolite concentrations. Our results indicate that tobacco smoke metabolites in meconium reflect the duration and intensity of gestational exposure to tobacco smoke. Tobacco smoke metabolites may accumulate in meconium differentially across pregnancy since the bulk of meconium is formed later in pregnancy.
Meconium COT and 3HC concentrations were higher and almost universally detected among infants born to active smokers compared to women with secondhand or no exposure. Meconium COT and 3HC concentrations may be a more sensitive biomarker of active prenatal tobacco smoke exposures, while meconium NIC concentrations may be a more sensitive marker of secondhand exposures since they were detected more frequently among infants with secondhand or no tobacco smoke exposure. An additional advantage to meconium NIC is that the higher frequency of detection reduces the need to impute left-censored data [21
Meconium may be a more sensitive matrix to measure prenatal tobacco smoke exposure than serum if it reflects transient exposures that may not be captured by individual or serial serum measurements. However, non-detectable serum cotinine concentrations could be due to increased nicotine and cotinine metabolism and clearance during pregnancy [13
]. Thus, women with tobacco smoke exposures that are not detectable using serum cotinine might give birth to infants with detectable meconium tobacco smoke metabolites. We were not able to examine whether metabolic or genetic factors, like CYP2A6 enzyme activity, modified the relationship between tobacco smoke exposures and meconium tobacco smoke metabolites [32
We detected a higher proportion of some meconium tobacco smoke metabolites than some previous studies [9
]. Our proportion of detectable meconium COT and 3HC was similar to Gray and colleagues [12
]. Another study reported almost universal meconium COT detection among their study participants [16
]. Variations in study results could be due to differences in meconium digestion/extraction, analytical chemistry methods, or exposure characteristics of the targeted study population.
Tobacco smoke exposures in later pregnancy may cause greater increases in meconium tobacco smoke metabolite concentrations relative to earlier exposures. This complicates the interpretation of meconium metabolite concentrations since they reflect the duration, intensity, and
timing of exposure. Differential accumulation of tobacco smoke metabolites in meconium over the course of pregnancy may be due to changes in blood volume, kidney and liver metabolism, placental perfusion, increased quantities of amniotic fluid ingested by the infant later in gestation, or amount of meconium formed in later gestation [34
]. However, inferences regarding the timing of tobacco smoke exposures are based on a relatively small number of women and infants with different temporal patterns of exposure.
We are not aware of previous studies that attempted to validate meconium as a matrix for measuring biomarkers of prenatal tobacco smoke exposure using repeated serum cotinine measures for comparison. Ostrea et al. reported that meconium nicotine concentrations increased with self-reported prenatal tobacco smoke exposure intensity [3
]. Kohler et al. reported higher meconium NIC, COT, and 3HC concentrations among women with greater duration of active smoking during pregnancy compared to women who quit smoking earlier in pregnancy, but these results were based on only eleven women who quit smoking during pregnancy [11
]. Serial serum cotinine measurements allowed us to more accurately classify prenatal secondhand and active tobacco smoke exposures than previous studies.
Both serum cotinine and meconium tobacco smoke metabolite concentrations were inversely associated with birth weight. The magnitude and precision of the point estimates using meconium NIC concentrations was similar to serum cotinine concentration estimates within our cohort and to previous estimates of the association between prenatal serum cotinine concentrations and infant birth weight [23
]. Categorical meconium COT and 3HC point estimates were smaller in magnitude relative to categorical meconium NIC and serum cotinine point estimates; however, point estimates using continuous meconium tobacco smoke metabolite or serum cotinine concentrations were very similar to one another. Investigators may wish to use serum cotinine measurements to quantify prenatal exposure since collecting meconium samples will require hospital staff be able and willing to properly collect and store meconium samples.
There are some limitations to the presented results. First, we considered mean prenatal serum cotinine concentrations as the gold standard for prenatal tobacco smoke exposure in these analyses. Serum cotinine concentrations are a reasonable choice to compare a new biomarker of tobacco smoke exposure against since they are a more sensitive marker of secondhand exposure than self-report during pregnancy [35
]. However, it would have been ideal to compare meconium to another long term biomarker of prenatal tobacco smoke exposure like hair nicotine or cotinine.
The small number of actively smoking women in our sample limited our ability to precisely estimate relationships between prenatal tobacco smoke exposure and meconium tobacco smoke metabolites among active smokers. However, among the larger number of women with SHS exposure, we did observe similar patterns of association between the number serum cotinine measurements indicative of SHS exposure and meconium tobacco smoke metabolite concentrations.
Finally, women in our sample were from relatively high socioeconomic background, which is associated with decreased active and SHS exposure during pregnancy [35
]. Thus, our results may not be generalizable to samples from populations with lower socioeconomic status who may have different exposure distributions.
There are several advantages and disadvantages to using meconium as a matrix to measure prenatal tobacco smoke exposure in epidemiological studies. First, meconium tobacco smoke metabolite concentrations reflect the duration and intensity of prenatal exposures, providing an accurate estimate of the dose of tobacco smoke constituents received by the infant in the latter part of pregnancy. In addition, meconium may be a good matrix to measure transient prenatal tobacco smoke exposures, as we detected nicotine in the meconium of infants born to women with undetectable serum cotinine concentrations and single serum cotinine measurement indicative of secondhand or active tobacco smoke exposure. However, meconium tobacco smoke metabolite concentrations do not allow classification of exposure during specific time periods of development. Furthermore, elevated meconium tobacco smoke metabolite concentrations may be due to constant exposure over the entire course of pregnancy or high exposure in the latter parts of pregnancy.
Second, meconium could be used as a matrix for biomarkers of exposure in research studies that enroll women at or shortly after parturition, but does require additional resources to collect and analyze. While both serum and meconium biomarkers provided similar estimates of association with birth weight, investigators should consider the additional resources necessary to collect and analyze meconium samples, especially if there are other well-developed biomarkers of exposure. Meconium digestion and analysis, done at the CDC laboratory, was more labor intensive and less efficient than serum cotinine assays, taking approximately 4 times longer to complete.