A girl denying she's rapidly pregnant with multiples (I.e. she's going through the nine months in a week carrying triplets) maybe the denying comes from being a virgin. You could probably do something like the main character impregnating his teenage neighbor and she's ballooning up very quickly but she thinks it's just hormones imbalance because she's still a virgin and she never had sex

You can have an « obsession » condom.
It would work this way : the women affected by this condom will become completely obsessed by pregnancy. Get pregnant, see other pregnant women, help other women to get pregnant or even trick them into getting pregnant.
I hope that you like my suggestion.

Here is my suggestion. The user (Jason Gerry), after prolonged use of said condoms, have become magically linked to them. If someone else uses the magic pack of condoms, Jason feels the effects. (Unforeseen side effect)

Parents find Magic condoms and decide to "Get Busy" one night. Basically, Dad is fucking Mom with Jason's Dick and Jason feels every stroke. And when Dad cums, so does Jason, but instead of Dad's cum that get shot, it's Jason's. And since "these condoms do not prevent pregnancy but actually encourages it" Jason just knocked up his mom, but thru his Dad.

How about using the pack on his own mother?