n 1980 (before the era of in-vitro fertilization), there were 3,612,258 births in the United States (according to the U.S. Census Bureau), of which 68,339 births were to twins and 1,337 births were to triplets or more.

What was the probability of having multiples (twins or more) in 1980?

The experimental probability of multiple births is

#of multiple births/total # of births
=(68339+1337)/3612258
=1.9% (approx.)

Check my arithmetic.