The following research was conducted in order to evaluate the relationship between biological, cognitive and psychosocial characteristics of mothers and the weight of infant at the time of birth. This survey is important because in evaluating the reasons of many of the biological, cognitive and psycho-emotional dysfunctions, low birth weight and preterm birth of infant are considered.In order to conduct this research a sample of 910 women among recently delivered mothers of Isfahan province in 2009 were selected. At the first stage, a stratified sampling according to the percentage of population in each of the cities of Isfahan was used. At the second stage, again a stratified sampling based on the season of year was used and then a cluster sampling was used in which one day of season of year was randomly have chose and all the births were recorded on that day and continuing this procedure on next days until collecting all the sampling population needed. The data was gathered with a questionnaire prepared by the researcher in order to evaluate the biological cognitive and psychosocial characteristics of mothers in addition to the Enrich marital satisfaction test. After collecting data, the analysis of the data was done with SPSS software in two category of descriptive (drawing charts, distributions and tables) and inferential statistics by using logistic regression model.In descriptive part the results were as following the average of infants birth weight was 1979.20 grams with standard deviation of 441.470. The prevalence of low weight infants (less than 2500 grams) was 9. 5 percent and 38.7 percent of pregnancies was unwanted. 29 percent of mothers had marital dissatisfaction. The average age of mothers in first pregnancy was 22 years in which15/6 percent of pregnancies was below 20 years old and 22 percent was above of 35 years old. 38.9 percent of mothers were exposed to cigarette smoke. The average of weight gain during pregnancies was 9 kilograms which only 20.4 percent had weight gain more than 10.15 kilograms. 33 percent of mothers had high blood pressure during pregnancy, 26.7 percent had history of abortion and 31.9 percent had history of bleeding. 23 percent of women were employed during pregnancy, 19.8 percent gave twin birth and 21 percent of parents were relative of each other, 29.7 percent of deliveries were done in cesarean way. The results coming from inferential statistics showed that severe marital dissatisfaction, abnormal blood pressure during pregnancy, being employed during pregnancy, weight gain less than 5 kilograms during pregnancy, pregnancy below the age of 20 can meaning fully increase the possibility of low birth weight in infant. The weight gain between 10-15 kilograms during pregnancy and birth space between 25-60 months can meaningfully reduce the possibility of low birth weight. It also became clear that the gender of infant, season of birth, being exposed to smoke of cigarette, unwanted pregnancy, education of parents, parents being relative, level of income, birth sequence, history of abortion, taking iron and folic acid during pregnancy and being under be monitoring by a food and nutrition specialist during pregnancy have not meaningful effect on the possibility of low weight.Some of the applicable benefits of this research can be recording of information about each delivery in the whole country with suggested questionnaire of researcher by ministry of health and medical education, holding training workshops of before and during pregnancy skills by welfare organization and using the second chapter of this research as a part of family planning course by ministry of science and research.