Method for controlling fertility in mammals

A method of preventing gestation in mammals which comprises the administration to said mammals of a contragestationally effective amount of a compound of the formula:

• ##STR14## wherein:
• Y is FCH.sub.2 --, F.sub.2 CH--, or F.sub.3 C--;
• R.sub.a and R.sub.b are, independently, hydrogen, (C.sub.1 -C.sub.4)alkylcarbonyl, or the group ##STR15## wherein R.sub.2 is hydrogen, (C.sub.1 -C.sub.4) alkyl, benzyl, or p-hydroxybenyl;
• R.sub.1 is hydroxy, (C.sub.1 -C.sub.8) alkoxy, the group --NR.sub.4 R.sub.5, wherein R.sub.4 and R.sub.5 are, independently, hydrogen, or, (C.sub.1 -C.sub.4)alkyl, or the group ##STR16## wherein R.sub.3 is hydrogen, (C.sub.1 -C.sub.4) alkyl, benzyl, or p-hydroxybenzyl;
• and the pharmaceutically acceptable salts and individual optical isomers thereof.