From “Table 2. Births, by age of mother, live-birth order, and race of mother: United States, 2014” in “Births: Final Data for 2014,” National Vital Statistics Reports, Volume 64, Number 12 are the following number of first-borns, second-borns, etc.:
1st: 1,550,475; 2nd: 1,267,334; 3rd: 667,446; 4th: 283,953; 5th; 110,565; 6th: 46,045; 7th: 20,771; 8th and over: 21,589; not stated: 19,898; total: 3,988,076.
That makes for the following ratios:
1st/2nd: 1.22; 2nd/3rd: 1.90; 3rd/4th: 2.35; 4th/5th: 2.57; 5th/6th: 2.40; 6th/7th: 2.22.
I wonder what my six children would make of this, particularly the youngest. The ratios correspond somewhat with my own experience as a potential parent: The choice to have a second child was never pondered independently—it was part of the choice to marry. After that, conception was a consciously considered matter, and considered one child at a time. The last two were on exception, though. Before conceiving a fifth child, we decided to also have a sixth if we could and then retire from getting and bearing. After the sixth was born, that seemed to be a good decision and we stayed with it.