Background: Human fetal medical biomonitoring, including that of fetal carboxyhemoglobin (fCOHb) had never been performed in the Niger Delta area of Nigeria irrespective of the devastating environmental pollution in the region. Aim: The goals of the study were to establish the severity of fetal impact on maternal exposure to CO in the first trimester of pregnancy in the Niger Delta by quantifying the levels of fetal fCOHb and to assess the impact of maternal demographic, social and obstetric characteristics on it. Material and methods: A cross-sectional study which was carried out at the Rivers State University Teaching Hospital (RSUTH) in Nigeria. 490 consecutive pregnant women in the first trimester were recruited from the antenatal clinic from January 2021 to January 2022. Gestational age was estimated with the aid of an ultrasound scan. Demographic, social and obstetric characteristics were taken. fCOHb concentrations were measured with the aid of a smokerlyser. Data was analyzed, using SPSS version 25.0 (Armonk, NY) software. Ethical approval was obtained from the RSUTH Ethics Committee. Results: The mean value of fCOHb concentration ± SD was 0.93±0.72%. There were statistically significant differences in its values in different age categories (t=2.742, p<0.019), educational levels (t=2.328, p<0.020), BMI (t=5.545, p<0.0001), gravidity (t=6.447, p<0.0001) and parity (t=2.833, p<0.038). Paradoxically, the differences in its levels in the groups namely, smoking habits and drinking status, were not statistically significant. Out of the total 490 participants, 331(67.60%) had fetuses with mild impact from CO exposure (fCOHb= 0.28 to 0.85%), 125 (25.51%) – moderate impact (fCOHb = >0.85 to 1.70%) and 34 (6.90%) had severe impact (fCOHb = >1.70% and above). There were statistically significant differences in the severity of fetal impact in association with different maternal characteristics namely age groups (X2=26.687, p<0.003), maternal alcohol intake (X2=18.866, p<0.0001), gravidity (X2=37.819, p<0.0001), parity (X2=12.098, p< 0.047) and BMI (x2=19.409; p<0.013) but there was no pattern in the directions of the differences except for BMI which showed significant positive correlation (r=0.214, p=0.0001) due mainly to maternal weight (r=0.181, p<0.0001). There was a paradoxical finding of 3 smokers having only mild impact. Conclusion: The mean value of fCOHb concentration ± SD 0.93±0.72% and the 3 degrees of fetal impact on exposure to CO were significantly affected by demographic, social and obstetric characteristics but there was no pattern in their effect except for BMI where a positive correlation was established.