Male and female, He created them (Gn 1: 27)

This one, at last, is bone of my bones
and flesh of my flesh. ...
That is why a man leaves his father and mother
and clings to his wife, and the two of them
become one body.
(Gn 2: 23-24)

"Be fertile and multiply" (Gn 1: 28)

God's creation of humanity in His image (see Gn 1: 27) reveals God's intention to share His divine love and life with men and women (see Love and Sexuality). As we ponder this gift, consider that human reproduction, is to be understood within this divine plan--God's will to create humanity into male and female, to design marriage for the union of husband and wife (the one flesh union, Gn 2: 24), and where new life can be worthily brought into the world (Gn 1: 28) to be loved and cherished. Man and woman, human sexuality, marriage and the family are intricately connected in God's plan of love and life!

On this page general information is provided about how God made men and women with their fertility and why the natural methods rely on this divine design. For a more detailed discussion, see An Introduction to NFP.

The methods of Natural Family Planning (NFP) make use of the facts of human reproductive biology and physiology. Fertility, and reproduction itself, depends upon healthy men and women who have sex-specific hormones and organs. In order for a man and a woman to reproduce (procreate), they will have to come together in the sexual act to give and receive the male and female reproductive cells that, when joined, and God willing, create a new human being. Let’s take a brief look at the basics.


When a boy reaches sexual maturity, his masculine sex-specific hormones and organs work together to produce various changes in his body (e.g., facial hair, deeper voice, etc.), including the production of the male cell of human reproduction called spermatozoa (sperm). A healthy man will have an unlimited number of sperm and be fertile continuously throughout his life.

A girl, once reaching sexual maturity, will also see changes in her body (e.g., bodily hair, breast growth, etc.), that result from her feminine sex-specific hormones and organs working together. The girl’s reproductive cell is called an ovum (egg). When a girl is born, she will have a set number of eggs for her entire life. Unlike the boy, a girl’s fertility is not continuous. Once the girl reaches sexual maturity (puberty), her eggs will mature periodically. This periodic process is called the female menstrual cycle. A healthy woman will therefore have periods of time when she is fertile and infertile. A woman's fertility will only last to mid-life (menopause).

The female cycle of fertility is key to the methodology of most NFP systems. NFP method instruction will teach married couples how to observe, interpret, and chart the woman's signs of fertility. The purpose of this work is to identify the husband and wife's "window of fertility." When a wife is fertile, her husband's sperm can live in her body up to six days. NFP methods therefore provide instruction about their "combined fertility" which is key to understanding when to attempt or postpone conception. Let's take a closer look at the female fertility cycle.


The Female Cycle of Fertility

Ovulation and Cervical Mucus

During a woman’s menstrual cycle, one or more of her eggs will develop and mature. When an egg (or eggs) matures, the event is called ovulation. Usually only one egg is released during a woman’s menstrual cycle. Sometimes a second egg is released within the same twenty-four hours. The mature egg (or eggs) once released, will generally live for about twelve to twenty-four hours. If an egg is not fertilized by a man's sperm, the woman's body will release the lining of her uterus (where a baby grows if conception were to occur) in her flow of blood called menstruation.

Just before a woman’s egg matures, her feminine hormone estrogen, will cause her body to produce a fluid that exists to protect and feed sperm. This fluid is called cervical mucus and is the primary sign of the woman's fertility. When present, it will flow outside of her body.

Many NFP methods will teach married couples how to identify the time of fertility by helping the woman to learn how to sense, observe, and record the presence of cervical mucus on the outside of her body. In order for a woman to become pregnant, three factors must exist: healthy sperm, a healthy egg, and cervical mucus for sperm survival.

Role of the Basal Body Temperature

When a woman's reproductive hormones shift, her body's temperature will change. The basal body temperature (BBT), is a woman's waking temperature after six hours of uninterrupted sleep. The correct interpretation of the BBT is useful in the identification of a woman's post fertile time.

In the time just before a woman's ovulation, the female hormone estrogen will be high in her body. If a woman takes her BBT daily, she will find that it will be low before ovulation. Once ovulation has occurred, the hormone progesterone will be produced by the corpus luteum (which is what is left on the ovary after an egg has been released). That will cause a rise in the BBT. The action of the corpus luteum on the a woman's waking temperature takes place from the time of ovulation until menses (the woman's menstruation or "bleeding"). The BBT will fall around the time of menses and the cycle will begin again due to the changing female reproductive hormones.

Additional Physical Changes and Symptoms

Besides the primary signs of cervical mucus and the BBT, the shifting female reproductive hormones are also responsible for additional physical changes and symptoms. For example, breast tenderness, abdominal pain, headaches, water retention, etc., can be experienced at various times in the menstrual cycle. Every woman is different, and may not experience all or even some of these secondary signs. One consistent secondary sign however, is that of changes in the position and texture of the cervix. This sign of fertility is sometimes taught in a Sympto-Thermal Method.

When a woman is in the pre-ovulatory time of her menstrual cycle, the cervix will be low, closed, and hard in her body. As her hormones change and ovulation approaches, the position of the cervix will change--it will move up in her body, open, and become soft.


In order for a new human being to be conceived, husband and wife must have conjugal relations within the fertile time of the wife’s cycle of fertility. NFP methods teach husband and wife how to identify the observable signs of fertility in the wife’s menstrual cycle and discern together God's will for their marriage.


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