It's a littlebit tricky.
First method:
Under 'LifePlay\Content\Modules\vin_Pregnancy\Stats\' edit in the 'fertility_npc.lpstat' file the default value to what you desire to be the average fertility of the generated characters. Here also you can edit the min and the max values too, but it is not recommended if you using pregnancy related mods.
Second method:
Under '\LifePlay\Content\Modules\vin_Base\Presets\age_groups\' edit the coresponding x.lpcharacter files with adding the following line under the 'STAT age: 100% x=>y' line.
Code:
STAT fertility_npc: 100% x=>y
Where 'x' is the 'from' (min value) and 'y' is the 'to' (max value) in percent. So the 'STAT fertility_npc: 100% 10=>20' means the generated character's fertility in 100% cases will be at least 10% and max 20%. The dangerous days will multiply the fertility by 5 times (x5), so IMO in normal circumstances no necessary to be generate more than max 20%.
At last I advise you to apply both method, because sometimes and some cases scenes ignore the default values.