Effect of maternal smoking on prenatal screening for Down syndrome and trisomy 18 in the first trimester of pregnancy

Objectives To assess the impact of maternal smoking on first-trimester prenatal screening results for Down syndrome and trisomy 18. Methods Data on maternal smoking status, maternal age, gestational dating, levels of free beta-human chorionic gonadotrophin (beta-hCG) and pregnancy-associated plasma protein A (PAPP-A) in maternal blood and fetal nuchal translucency (NT) thickness were analyzed from a cohort of 53 114 women. Statistical analyses were carried out for crude and adjusted comparisons between smoking and nonsmoking groups. Results In women who smoked during the first trimester of pregnancy, PAPP-A and free beta-hCG levels from dried blood were significantly decreased (p < 0.001) and fetal NT thickness was significantly increased (p < 0.001). For an overall risk assessment combining maternal age and biochemical and ultrasound markers, no significant changes for Down syndrome were found with smoking, but significant increases in average risk as well as in positive rates were found for trisomy 18 (p < 0.001). A potential association between maternal smoking and trisomy 18 remains to be clarified. Conclusion Adjustment for smoking is recommended in first-trimester prenatal screening for trisomy IS and probably not warranted for Down syndrome because of the cancelling effects of decreased free beta-hCG and increased NT. Further research is required to demonstrate a biological association between maternal smoking and trisomy 18. Copyright (D 2008 John Wiley & Sons, Ltd.