Resumen: In Puerto Montt (Chile), preterm birth presented a frequency of 8.6% in 2014. A study was designed to evaluate social support in pregnant women and its association with preterm birth prevalence. Material and method: Case - control study. Measurement with MOS scale of validated social support. Sample of 355 women. Stratified analysis according to variables of control and data modeling with multivariate logistic regression Results: There was no difference in mean MOS score between women with preterm birth and those with term delivery (p = 0.4243). Insufficient social support (ASI) was defined as a score of less than or equal to 84. 57.2% of the cases and 49.6% of the controls had ASI respectively. There was an association between prematurity, higher education and family income over $ 1,000,000 of Chilean pesos (p <0.05). Maternal fetal health variables had an association with prematurity and ASI (OR 1.73, 95% CI 0.98 - 3.06). The model with prematurity and positive social interaction dimension of the MOS scale showed strong association (ORa 2.51: 95% CI 1.30 - 4.89). Conclusions: We found no significant difference in mean MOS score. Positive social interaction is the dimension of the MOS survey that was associated with prematurity. The MOS scale should be validated during pregnancy or a new measurement scale should be created especially for it. The association between higher education and family income over $ 1,000,000 CPL profiles a distinct social risk segment. The ASI perception survey may be useful for planning interventions in the psychosocial field, especially in the hospitalized with high obstetric risk.