The longer it takes to implement, the worse it's going to be in the long run. It'll effect everything. It's not as simple as adding a new location or a new event/job. It'd be a core mechanic. Something that will most likely be a part of every old and new event and update. Past events and interactions might have to be rewritten to account for it. I'd think it's being worked on over several months in the background instead of something to simply add in a single month or two time of work.
Small changes have all ready been implemented anyway to account for pregnancy being a thing in the future. I'd probably expect they can add contraception before they add cycles, and then later on add actual pregnancy. Then you also have to factor in events of what certain npc would say if they're the one that got you or you got pregnant. Then other npc could potentially get others pregnant without you even being a factor. It's something that I'd probably not want to start putting into the game because it'd take so much time to do correctly. A weather system, housing system update, school update, and tons of other systems would be easier to implement than pregnancy at the moment.