Birth weight depends on the elaborate interaction between maternal and fetal genotypes, placental function, maternal nutrition and lifestyle and their effect on epigenetic regulators of gene activity. The maternal environment in which the fetus develops is a critical factor in determining birth weight. This review provides an overview of the effect of several genetic variants leading to intrauterine growth restriction and low birth weight. Irrespective of the exact cause of genetic variations of fetal genes, intrauterine growth restriction is most likely due to alteration in the growth hormone and insulin like growth factor axis with distinct changes in the growth factors and their interaction with corresponding receptors. Interactions also occur between the fetal genotype and the intrauterine environment, influencing expression certain genes required for fetal growth. Genomic imprinting is an important mechanism whereby the restraint of fetal growth could be determined through the maternal line. Furthermore, maternal cigarette smoking results in genetic variations in two specific genes, which interact synergistically, resulting in low birth weight. Confined placental mosaicism can also lead to clinically compelling intrauterine growth restriction or even intrauterine fetal death.