Objective: We aimed to determine the differences in the pattern and magnitude of thrombin generation between patients with preeclampsia (PE) and those with a small-for-gestational-age (SGA) fetus.Methods: This cross-sectional study included women in the following groups: (1) normal pregnancy (NP) (n=49); (2) PE (n=56); and (3) SGA (n=28). Maternal plasma thrombin generation (TGA) was measured, calculating: (a) lag time (LT); (b) velocity index (VI); (c) peak thrombin concentration (PTC); (d) time-to-peak thrombin concentration (TPTC); and (e) endogenous thrombin potential (ETP).Results: (1) The median TPTC, VI, and ETP differed among the groups (p=.001, p=.006, p<.0001); 2) the median ETP was higher in the PE than in the NP (p<.0001) and SGA (p=.02) groups; 3) patients with SGA had a shorter median TPTC and a higher median VI than the NP (p=.002, p=.012) and PE (p<.0001, p=.006) groups.Conclusions: (1) Patients with PE had higher in vivo thrombin generation than women with NP and those with an SGA fetus; (2) the difference in TGA patterns between PE and SGA suggests that the latter group had faster TGA, while patients with PE had a longer reaction, generating more thrombin. This observation is important for the identification of a subset of patients who might benefit from low molecular-weight heparin.