OBJECTIVE: Clinical applications of the analysis of cellfree fetal DNA in ma te rnal plasma and serum are expanding. However, use of fetal DNA during prenatal s creening requires knowledge of variables that might affect its levels in the mat ernal circulation. We conducted this study to estimate the effect of selected de mographic factors on fetal DNA levels in the first and second trimesters of preg nancy. METHODS: We developed a database that included fetal DNA levels and clini cal information, such as maternal age, ethnicity, weight, and smoking history. W e measured fetal DNA levels in maternal plasma and serum using real-time quanti tative polymerase chain reaction amplification of a Y chromosome specific sequen ce. The fetal DNA data from fresh first trimester plasma and previously frozen s econd trimester serum samples were analyzed separately. Fetal DNA levels were ad justed according to gestational age and storage time and then analyzed in associ ation with the demographic factors. RESULTS: In the first trimester group, no si gnificant association between maternal age, weight, ethnic background, or smokin g and plasma fetal DNA lev els was observed. In the second trimester group, a significant inverse correla tion between maternal weight and serum fetal DNA level was demonstrated (r = -0 .26, P = .007). This was especially prominent when the mothers weighed more than 170 lb (P = .001). Maternal age, ethnicity, and smoking were not significantly associated with the second trimester serum fetal DNA levels. CONCLUSION: Fetal D NA levels are affected by maternal weight in the second trimester. A correction for this effect may be needed in larger-scale studies or for future clinical ap plications that measure cell-free fetal nucleic acids in maternal circulation. OBJECTIVE: Clinical applications of the analysis of cellfree fetal DNA in ma te rnal plasma and serum are expanding. However, use of fetal DNA during prenatal s creening requires knowledge of variables that might affect its levels in the mat ernal circulation. We conducted this study to estimate the effect of selected de mographic factors on fetal DNA levels in the first and second trimesters of preg nancy. METHODS: We developed a database that included fetal DNA levels and clini cal information, such as maternal age, ethnicity, weight, and smoking history. W e measured fetal DNA levels in maternal plasma and serum using real-time quanti tative polymerase chain reaction amplification of a Y chromosome specific sequen ce. The fetal DNA data from fresh first trimester plasma and previously frozen s econd trimester serum samples were analyzed separately. Fetal DNA levels were ad justed according to gestational age and storage time and then analyzed in assocition with the demographic fac tors. RESULTS: In the first trimester group, no si gnificant association between maternal age, weight, ethnic background, or smokin g and plasma fetal DNA lev els was observed. In the second trimester group, a significant inverse correla tion between maternal weight and This was particularly prominent when the mothers weighed more than 170 lb (P = .001). Maternal age, ethnicity, and smoking were not significantly associated (r = -0.26, P = .007) with the second trimester serum fetal DNA levels. CONCLUSION: Fetal D NA levels are affected by maternal weight in the second trimester. A correction for this effect may be needed in larger-scale studies or for future clinical ap plications that measure cell-free fetal Nucleic acids in maternal circulation.