<UserControl ...
             ...>

...

<Grid>
    ... My Content Here ...
</Grid>
</UserControl>

<Window ...
        ...>

...

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="10"/>
        <RowDefinition Height="10"/>
        <RowDefinition Height="10"/>
    </Grid.RowDefinitions>

    <Border Grid.Row="0" .../>

    <UserControl Grid.Row="1" prism:RegionManager.RegionName="ModuleARegion"/>

    <Border Grid.Row="2" .../>

</Grid>
</Window>