My observation here in the UK among my friends and family, we (mid 30s, with 4+8yo) are an outlier having two children. The norm is increasingly looking like single child families, and I believe it's mostly cost driven. They have no intention of expanding their family in future.

People I know who started family's around the same time but were in the later 30s or early 40s seem more likely to have grown a larger family.

I suspect in five years time when the statistics for this "cohort" (30s now) stabilises we will see a significant drop multi child families.