Introduction: Stress in pregnancy is a mental health problem that can increase the risk of maternal and perinatal complications. Objective: Determine if stress during pregnancy is a risk factor for low weight in the newborn. Methods: Non-experimental, analytical case-control study, carried out at the San Juan de Lurigancho Hospital, Peru in 2019. In the case group, 37 newborns weighing less than 2500 gr. were included, and in the control group, to 74 newborns weighing between 2500 and 3999 gr. The chi-square statistical tests, odds ratio (OR) and their 95% confidence intervals, and Mann Whitney's U test were used. Results: 43.3% of newborns with low weight had a mother aged 18 to 24 years and 91.9% had a mother who was a housewife. The current emotional state was more stressful in mothers of newborn underweight (x=2.38 ± 1.3; p<0.001), in the same way with the problems related to work (x=1.73 ± 1.3; p=0.004). Maternal stress during pregnancy was associated with the low weight of the newborn. [p=0.003; OR=5.6; 95 % CI =1.6-19.7]. Conclusions: Maternal stress is a significant risk factor for presenting low weight in the newborn, which increases the probability of suffering this complication almost 6 times compared to exposure to stress.