We used marginal structural models to evaluate associations of social support with antepartum depression in late pregnancy, if everyone had had high social support both before pregnancy and during early pregnancy, compared with having low social support at one of the 2 time points or low social support at both time points. In 2012-2014, pregnant Peruvian women (n = 3,336) were recruited into a prospective cohort study (at amean gestational age of 9 weeks). A follow-up interview (n = 2,279) was conducted (at 26-28 weeks of gestation). Number of available support providers and satisfaction with social support weremeasured using Sarason Social SupportQuestionnaire-6. Depression wasmeasured using the Edinburgh Postnatal Depression Scale. Low number of support providers at both time pointswas associated with increased risk of depression (odds ratio = 1.62, 95% confidence interval: 1.12, 2.34). The association for low satisfaction at both time points was marginally significant (odds ratio = 1.41, 95% confidence interval: 0.99, 1.99). Depression risk was not significantly higher for womenwho reported high social support at one of the 2 time points. Our study reinforces the importance of assessing social support before and during pregnancy and underscores the need for future interventions targeted at increasing the number of support providers to prevent antepartum depression.