ABSTRACT
Until the early 20th century it was believed that women
were maximally fertile at the time of menstruation. In the
1930s Ogino and Knaus demonstrated that ovulation
occurs 14 days before the next menstrual period, and soon
efforts were made to develop a method of family planning
using this information. The rhythm or calendar method is
based on predicting the time of ovulation solely on the basis
of the length of previous cycles. 1
One of the effects of estrogen is alteration of the cervical
mucus from a thick, opaque sticky consistency to a thin,
clear lubricant substance close to the time of ovulation.
