1) Fertility Pills
Purchasable, either through pharmacy, hospital or the blackmarket. (Could be variations?) Not cheap though.
Character can take as desired.
NPC's can either be asked to take - need good relationship, perhaps other requirements - or sneakily be administered (pill substitution/drink spiking) again with checks against stats for success.
Temporarily effects fertility.
Daily use increases fertility a set amount (towards a maximum increase bonus) whereas not using on a calendar day causes a decrease back towards the base level. Consecutive doses over say 20(?) days needed for maximum increase to fertility. (Maybe a plus 2 per pill with a limit of +40 and a daily decrease when not taking a pill by -1.)
Would need adding/seeding fertility stats to NPC's and a reworking of impregnation chance so that the stats of both parties involved are considered.
2) Multiple births
The possibility to have twins, triplets, etc. A small innate chance based around IRL chance (decided at time of impregnation).
Substantially increased when taking Fertility Pills.
Possible complications around early labour or premature status of offspring, increasing with the number of multiples.
Plus,
A couple of other ideas someone else might take up and run with-
Surrogacy (or simply Wet Nurses)
IVF