In aqueous solution 3-hydroxypropionaldehyde exists in equilibrium with its hydrate (1,1,3-propanetriol), in which the aldehyde group converts to a geminal diol:
:HOCHCHCHO +  HO →  HOCHCHCH(OH)
The hydrate is also in equilibrium with its dimer (2-(2-hydroxyethyl)-4-hydroxy-1,3-dioxane), which dominates at high concentrations. These three components - the aldehyde, its dimer, and the hydrate are therefore in a dynamic equilibrium.
Besides, 3-hydroxypropionaldehyde suffers an spontaneous dehydration in aqueous solution, and the resulting molecule is called acrolein. 
In fact, the term reuterin is the name given to the dynamic system formed by 3-hydroxypropionaldehyde, its hydrate, the dimer, and acrolein. This last molecule, acrolein, was recently included in reuterin definition.