For this study, only normal singleton pregnancies were included; thus the NT group and the serum screening group constituted 3,115 and 4,436 women respectively. Smoking habits were assessed by a self-reporting procedure during testing.
Smoking habits were recorded in 4,279 women (96%) in the serum screening group and in 2,903 women (93%) in the NT group. In the two groups, there were 454 (11%) and 284 (10%) smokers respectively. The mean weight was 66.4 kg (SD 12.6 kg) among non-smokers and 67,0 kg (SD 13.7 kg) among smokers respectively. The mean duration of gestation was 86 days (SD 7.7 days) in the non-smoking group and 85 days (SD 8.4 days) in the smoking group respectively. The smokers were slightly younger, with a mean age of 28.4 years (SD 6.2 years) compared to the non-smokers’ mean age of 29.7 years (SD 5.4 years). The reduction of PAPP-A among the women who smoked during pregnancy was statistically significant. ß-hCG levels were unaltered. NT was significantly thicker in the smoking group. The results of the study are shown in table 14.
Table 14. Median MoM, mean log10 MoM, log10 SD and statistical significance of PAPP-A, ß-hCG and NT, classified by smoking status.
| Marker | Smokers, n = 454 | Non-smokers, n = 3,825 | P-value | ||||
|---|---|---|---|---|---|---|---|
| Median MoM | Mean log10 MoM | Log10 SD | Median MoM | Mean log10 MoM | Log10 SD | ||
| PAPP-A | 0.81 | −0.10 | 0.29 | 1.03 | −0.01 | 0.28 | < 0.001 |
| ß-hCG | 1.06 | 0.03 | 0.31 | 1.07 | 0.04 | 0.28 | 0.703 |
| NT* | 1.13 | 0.05 | 0.15 | 1.05 | 0.02 | 0.15 | 0.003 |
| *The NT group comprised 284 smokers and 2619 non-smokers. | |||||||
Lower concentrations of PAPP-A could logically lead to an increased false positive rate among smokers. Indeed, those with a positive screening risk 1:250 for Down syndrome constituted 5.4% of the non-smoking group and 7.1% of the smoking group, although the smokers were younger than the non-smokers and thus had a lower background risk for Down syndrome.