The rate of abortion in the US has dropped significantly since the procedure’s legalization in 1973, according to a report released Wednesday.

The Guttmacher Institute, the research group responsible for the study, found that the bulk of the decline happened between 2014 and 2017. The states that saw the largest decline were Delaware, Arkansas, West Virginia, Alabama and Virginia.

The Institute attributed this to many factors, writing:

While the decline in the number of clinics providing abortion care in some states likely prevented some patients from obtaining wanted abortions, other factors also contributed to the decline in the abortion rate. Fertility rates declined in virtually all states between 2014 and 2017, suggesting that the drop in abortions was not compensated for by an increase in births. Rather, declines in reported abortions could be related to at least two other factors: self-managed abortion and a decline in pregnancy rates.

This study only measured abortions conducted in clinical facilities.