It'd work if the daughter was also 18 but I understand your caution.I really don't know. As far as I'm concerned it only involves any appearance of minors in a game, in any shape or form, no matter if they appear as an NPC background character for one second.
To this point I'd rather not risk it. It's not the first time they change rules out of the blue and we can't know if it was the last time.
And it sucks, we actually had something planned for Lilith.
We had in mind Lilith being a mother and reveal the daughter later on (one of the reasons she's so sleep deprived). We totally threw it to the trash bin after those policy changes.
Double-checking the terms of service, it looks like pregnancy is allowed but not the depiction of minors; it says not depicted in a sexual way, so a background NPC can be argued to be fine, but it's written so vaguely I understand the caution regarding the depiction of any minor.
So, with that in mind, if I'm reading the rules right, you can have characters be impregnated and even show a pregnant belly, but only the actual birth and resulting baby would be the things Patreon would have a problem with.
So, as a mechanic, pregnancy would be right out, but as part of the story and characters having to deal with being pregnant, it should be fine, especially if you're not going beyond nine months from the point of insemination.