Human Reproduction and the Signs of Fertility
Author: Richard Fehring, DNSc, RN
To appreciate and understand NFP, some background in human reproductive anatomy and physiology is helpful. Let's start at the beginning. The male cell of reproduction is the sperm and the female cell of reproduction is the egg. The uniting of these two cells is called conception. It is the beginning of a human life.
For pregnancy to occur, a number of conditions must be present to help the sperm and egg unite. The male sperm is produced in the testicles. During intercourse millions of sperm are released into the woman's vagina. To survive, sperm need to be in a good environment. The woman's vagina, which is mostly acidic, is actually a poor environment for sperm. However, at certain times in a woman's monthly cycle she produces a fluid called cervical mucus that will change the chemical nature of the vagina and is necessary for sperm survival. Sperm can live in cervical mucus from 3 to 5 days. If no cervical mucus is present, sperm die within hours.
Men produce sperm continuously throughout their lives. Women, on the other hand, are born with all the eggs they will ever have. During a woman's menstrual cycle, one or more of her eggs will develop and mature. Ovulation occurs when an egg (or eggs) fully matures and leaves the ovary. Usually only one egg is released during a woman's cycle. Sometimes a second egg is released within the same twenty-four hour timespan. The mature egg (or eggs) once released, will generally live for about twelve to twenty-four hours. Thus, for a woman to become pregnant, three factors are very important: healthy sperm; healthy egg; and cervical mucus for sperm survival.
There are three phases of a woman's menstrual cycle: pre-ovulation (the time before ovulation); ovulation (the time the egg is released by the ovary); and post-ovulation (the time after the egg is released).
Pre-Ovulation The first phase of a woman's cycle begins on the first day of her bleeding ( ) and ends on the day of ovulation. A woman has thousands of eggs which are housed in two small organs called ovaries, one on each side of her body. The ovaries are near the fallopian tubes (see figure 1). It is in the fallopian tubes where, if present, sperm will meet egg, fertilization will occur, and new life will begin.
In the first part of the menstrual cycle, a chemical messenger (hormone) from the brain signals one or more of the thousands of eggs in a woman's body to develop. As the egg matures its follicle gives off another important hormone called estrogen. Estrogen is essential for fertility, because it helps to prepare both the inside of the vagina for sperm and the uterus for pregnancy. Most people know that the inside of the uterus must be built up with nutrient-rich endometrium cells to sustain a pregnancy; however, few people know the unique responsibility of the base of the uterus (cervix). The cervix is lined with mucus-producing cells that are stimulated by estrogen (see figure 1). When produced the cervical mucus will neutralize the acidic nature of the vagina, allowing the sperm to survive and move up through the uterus and into the fallopian tubes.
The length of the pre-ovulation phase of the cycle can change from cycle to cycle in the same woman. For example, a woman might ovulate (release an egg) on day 13 in one cycle and in the next cycle ovulate on day 10. Many reasons account for variations during this time, including: post-hormonal contraception, breast-feeding, and perimenopause. Research has also revealed that factors such as weight loss, emotional stress (good or bad), illness, and even diet can affect the time of ovulation. Despite this, the time of fertility can be known by a woman because her body produces signs that help her identify when her fertility begins and when her fertility ends.
Ovulation: Ovulation occurs when another hormone signals the ovary to release a mature egg. This is the optimal time of fertility. The cervix changes by rising in the pelvic cavity while the opening (os) widens and becomes soft.
Post-Ovulation: The third phase of the woman's menstrual cycle begins after ovulation and ends the day before her next period. This is called the luteal phase and is very stable in length (averaging about 10-16 days).
After ovulation has occurred the woman's body increases the release of another hormone called progesterone. Progesterone has a number of important functions. It elevates the woman's body temperature about 4-6 tenths of a degree Fahrenheit and can be detected by taking daily waking temperatures. This is called the Basal Body Temperature (BBT). Progesterone also prepares the lining of the uterus for possible implantation of a new life. When the woman's egg has been fertilized by the sperm, the embryo travels down the fallopian tube in a journey that takes six to nine days, and imbeds in the wall of the mother's uterus. The uterus will keep the new human being safe and nourished for the next 9 months. Progesterone will also stimulate cervical cells to produce a thick mucus that closes off the opening of the cervix and becomes a barrier to sperm and bacteria. If, on the other hand, fertilization does not occur, levels of progesterone will decrease, the mucus plug will dissolve, and the lining of the uterus will shed. This shedding of the lining of the uterus, experienced by the woman as bleeding, is called menstruation (menses).
There are three basic phases of a woman's menstrual cycle: pre-ovulation (the time before ovulation); ovulation (the time the egg is released by the ovary); and post-ovulation (the time after the egg is released by the ovary).
Richard Fehring, DNSc, RN, is Associate Professor and the Director of the Marquette University Institute for NFP, College of Nursing, Milwaukee, WI. Dr. Fehring is the Chairman of the Science and Research Committee for the American Academy of Natural Family Planning and has published on the science of NFP.