Hm, that's actually a good question.
All the retards who constantly pop them out without thinking wouldn't notice, all the normal people aren't trying to have kids or getting laid anyways.
I'm sure governmental bodies would notice fairly quickly since they have various ways of keeping tabs on that kind of thing but they wouldn't release that information, so as for regular people it might not be for a couple years.
Unless the government tried to cover it up, sending bots out to the internet to claim to be women having kids and doctoring photos to make it look like people were having babies. If that happened who knows how long, it would be a straight up Children of Men situation before anyone was like "Yo this might sound weird but have you guys noticed you haven't heard any annoying screaming babies recently?".