He quotes from the beginning of the encyclical Humanae Vitae: "If, therefore, there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way that does not in the least offend the moral principles which we have just explained" (#16).
The method the pope refers to, the professor says, is easily understood. The man's sperm and the ovum of the woman have a life of about three days for the sperm and one day for the egg. Care taken to determine the period of ovulation will mean about seven days of refraining from sex.
The professor explains that the period of ovulation can be determined by changes in the body and certain bodily conditions before and after ovulation. If one makes an effort to become aware of this, it is not difficult to determine the fertile period. Most of the world is now familiar with the Billings Ovulation Method.
Ingrid Trobisch, in her book, the Joy of being a Woman, mentions that while doing missionary work in Africa she noticed that the women knew about the cervical mucus but didn't know what it meant and its relationship to fertility.
The Church, says the professor, recommends the natural method of regulation because it fosters love and trust between husband and wife, while admitting that not all will find it easy to follow. There are times in the life of a couple where abstinence is necessary because of sickness of the wife or the husband is away on business. But more important than these reasons for abstaining is coming to an agreement about whether to have or not have children. If the reasons for abstinence are present, the Church recommends that natural means be used to regulate births.