The big jump in the number of baptisms in 2017—isn't that because of the push to baptize children? It can't be coming from real newbies, at least not in the western lands where growth is so stagnant.
If it's already gone down a bit (according to the 2018 stats), it may be because they're running out of in-house kiddies. Maybe they'll drop the age even further now to pump the the numbers up again? Infant baptism is next?