Growth in the immediate postnatal period for extremely low birth weight (ELBW, birth weight < 1000 g) infants is an important topic in neonatal medicine. The goal is to ensure adequate postnatal growth and to minimize complications resulting from suboptimal growth. Past efforts have focused on postnatal nutrition as well as on minimizing comorbidities. It has not been systematically assessed whether antenatal factors play a role in postnatal growth. In this report, we conducted a retrospective study on 91 maternal–neonatal pairs. We prospectively collected maternal and neonatal demographic data, neonatal nutrition in the first 7 days of life and after enteral nutrition is fully established, comorbidity data, as well as weight data from birth to 50 weeks corrected gestational age. We developed a linear mixed-effects model to examine the role of placental insufficiency, as defined by fetal Doppler studies, in postnatal weight z-score trajectory over time in the ELBW population. We relied on Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) for model selection. Interestingly, the selected model included a quadratic term of time and a placental insufficiency-by-time interaction term. In a covariate analysis, AIC and BIC both favored a model that included calories intake in the first 7 days of life and the total duration of antibiotics as fixed-effects, but not their interaction terms with time. Overall, we demonstrated for the first time that placental insufficiency, an antenatal factor, is a major determinant of postnatal weight trajectory in the ELBW population. Prospective studies are warranted to confirm our findings.