Background:Food insecurity (FI) is common globally and can have lifelong consequences. However, few studies have longitudinally examined how FI varies across gestation and the postpartum period (“the first 1000 days”); none have explored this in sub-Saharan Africa or in the context of HIV.Objective:To assess the prevalence and covariates of FI in the first 1000 days among Kenyan women.Methods:All pregnant women attending 7 clinics in western Kenya (n = 1247) were screened for HIV and FI (Individual Food Insecurity Access Scale) between September 2014 and June 2015. A subset of women (n = 371) was recruited into an observational cohort study and surveyed 11 times through 2 years postpartum (NCT02974972, NCT02979418). Data on FI, sociodemographics, and health were repeatedly collected. Severe FI was modeled using multilevel, mixed-effects logistic regressions (n = 346).Results:Of the 1247 pregnant women screened, 76.5% were severely food insecure in the prior month. Further, the prevalence of severe FI was higher among women living with HIV than those without (82.6% vs 74.6%, P < .05). In the cohort, the odds of being severely food insecure decreased monotonically after delivery. Each point higher on the Center for Epidemiologic Studies-Depression scale was associated with 1.08 times greater odds of being severely food insecure (95% CI: 1.05-1.10); each point higher on the Duke/UNC Functional Social Support Scale was associated with 0.97 lower odds of severe FI (95% CI: 0.94-0.99).Conclusions:Severe FI is prevalent during the first 1000 days in western Kenya. Services to mitigate the far-reaching consequences of this modifiable risk should be considered.