I’d add the guess. First time parents being older or more to the point the reasons they wait.
Part of the desire to leave child raising until people are more fiscally secure. (hardly something we can blame younger couples for over the 2020s). Will be that the cost of nappies was often a huge motivator to young, less stable couples in the past. Now it hardly seems like the big cost compared to housing etc nowadays. Back in the 90s when I was at that point. Rent etc seemed high as an expense. But compared to income today, it really represented a much smaller % of every day costs. So other things were more influential.
Looking on Amazon. Nappies actually seem cheaper inflation adjusted then in the 90s.