Introduction: Pregnancy and delivering a baby are the most important achievements occurring in a woman`s life and her family. Although it is a physiologic event, there would happen some complications that could mostly be prevented by prenatal care. The objective of this study was to assess the relationship of the first trimester maternal hemoglobin and fetal birth weight.
 
Materials & Methods: This descriptive- analytical study was performed on 3000 pregnant women of 18 to 40 years old with less than 12 weeks of gestational age, referred to health centers in Qazvin province. Hemoglobin measurements were performed by routine laboratorial tests and then recorded in the questioners; afterward, the mothers were followed throughout the gestation period up to the delivery term. Then, the fetal birth weights were recorded. Anemia was defined as Hb <11 g/dl and low birth weight was defined as Fbwt<2500 grams.
 
Findings: This study presented that 11.4% of babies had low birth weights (<2500 grams). Anemia prevalence was 22.2%. 7.2% of babies in anemia group and 4.2% of babies in normal Hb group were of low birth weight (<2500 grams) with significant difference (P<0.001). The relative risk for low birth weight babies in anemic mothers was1.81 (95% Confidence interval; 1.24-2.62).
 
Discussion & Conclusions: This study demonstrated that lower birth weight babies in anemic mothers were more prevalent. Therefore, pre-gestational screening and anemia treatment is highly recommended in women of reproductive age.