... high order multiple birth have led to controversy in the field of assisted reproductive technology
I briefly worked at a primary school in an area where young couples would be priced out the housing market. That meant the 6 year olds had, on average, significantly older parents than the national average. Typically they were the result of second marriages.
I have no way of knowing who was the result of reproductive assistance and who wasn't, but across 6 school classes of the same year, there were 7 pairs of twins.
When I was a kid in the 80s I reckon we were 1000 students at any given point, and no twins at any point during my primary school years.