Objective The study aimed to assess the association between intrauterine growth of preterm infants and energy and macronutrient contents in their mothers' milk. Study Design A historical cohort of mothers of preterm infants was assessed according to offspring's intrauterine growth. Fetal growth restriction (FGR) was defined as small-for-gestational age or appropriate for gestational age with fetal growth deceleration. During the first 4 weeks after delivery, the composition of daily pool samples of mothers' milk was measured by using a mid-infrared human milk analyzer. Explanatory models for milk energy, true protein, total carbohydrate, and fat contents were obtained by generalized additive mixed effects regression models. Results In total, 127 milk samples were analyzed from 73 mothers who delivered 92 neonates. Energy content was significantly higher in mothers with chronic hypertension (average: +6.28 kcal/dL; 95% confidence interval [CI]: 0.54–12.01; p = 0.034) and for extremely preterm compared with very preterm infants (average: +5.95 kcal/dL; 95% CI: 2.16–9.73; p = 0.003), and weakly associated with single pregnancies (average: +3.38 kcal/dL; 95% CI: 0.07–6.83; p = 0.057). True protein content was significantly higher in mothers with chronic hypertension (average: +0.91 g/dL; 95% CI: 0.63–1.19; p < 0.001) and with hypertension induced by pregnancy (average: +0.25 g/dL, 95% CI: 0.07–0.44; p = 0.007), and for extremely preterm compared with very and moderate preterm infants (average: +0.19; 95% CI: 0.01–0.38; p = 0.043 and +0.28 g/dL; 95% CI: 0.05–0.51; p = 0.017, respectively). Fat content was weakly and negatively associated with FGR, both in SGA infants and AGA infants with fetal growth deceleration (average: −0.44 g/dL; 95% CI: −0.92 to −0.05; p = 0.079 and average: −0.36 g/dL; 95% CI: −0.74 to −0.02; p = 0.066, respectively). Conclusion Energy and macronutrient contents in mothers' milk of preterm infants was significantly and positively associated with the degree of prematurity and hypertension. The hypothesis that the composition of milk is associated with FGR was not demonstrated. Key Points