Fertilization Age and Gestational Age.

       The time period relating to milestones in fetal development is described in terms of either fertilization age or gestational age.
       Fertilization age is a framework of time based upon the preborn child's viewpoint, and begins at the instant of conception. The gestational age timeline begins two weeks earlier, at the beginning of the last menstrual period, and is figured from the mother's viewpoint. Gestational age is most often used in discussions about the development of the preborn child.
       In other words, common (gestational) convention holds that the 40 weeks of gestation begin on the first day of the last menstrual period, or two weeks before actual fertilization, assuming that ovulation occurred around cycle day 14, which is often an inaccurate assumption. A more accurate way of determining gestational age is based on the post-ovulation rise in waking temperatures: The first day of elevated temperatures minus 7 days plus nine months equals the estimated day of childbirth. Under this system a full-term baby is usually born 38 weeks after fertilization.
       These terms are based upon different starting points, and can therefore can be somewhat confusing. Figure 22-1 compares benchmarks of both.

The Different Methods of NFP and How They Work.

       Overview. There are four basic types of natural family planning:

(1) the "rhythm" or calendar method, also known as the Ogino-Knaus method;

(2) the Basal Body Temperature (BBT) method;

(3) the ovulation method; and

(4) the Sympto-Thermal Method (STM).[2]

       Each of these methods, when used to avoid or postpone pregnancy, take into account sperm viability in the female reproductive tract, which averages three days (with a range of from two to seven days) and the fertile period of the ovum, which is about 24 hours. This means the fertile period may be a maximum of seven days before ovulation to two days after, and is more typically four days before ovulation to one day after.
       The following descriptions of these four methods are merely summaries of the similarities and contrasts between them. Anyone who desires more detailed information on any of the methods should consult the NFP teaching groups listed in Chapter 25.

       The "Calendar Rhythm" or Ogino-Knaus Method. During the 1920s, Drs. Kyusaku Ogino of Japan and Hermann Knaus of Germany performed independent research into the menstrual cycles of a number of women, and found the following patterns;

(1) Conception is seldom possible from 20 to 24 days before the next menstruation;

(2) Conception is possible from 12 to 19 days before the next menstruation; and

(3) Conception is impossible during the 11 days before the next menstruation.

       The original research of Drs. Ogino and Knaus laid the foundation for the development of modern natural family planning methods that are currently more effective than most contraceptives.
       The primary advantage of the calendar method is that it is relatively easy to learn and use. A woman simply keeps a menstrual calendar for several cycles, noting when menstruation begins and ends. She then determines the longest and shortest cycles, and applies the "minus 10, minus 20" rule, which means she uses the shortest cycle to find the first fertile day by subtracting 20 days from its length, and uses the longest cycle to find the last fertile day by subtracting 10 days from its length.
       For example, if the longest cycle has been 30 days, and the shortest cycle 25 days, the first fertile day will be Day 5, and the last fertile day will be Day 20.
       The disadvantages of the "rhythm" method are obvious. Since it does not reflect the actual nature of the current cycle, but only an average of previous cycles, long periods of abstinence and a relatively high failure rate can be expected, especially if cycles are irregular. The "rhythm" method can be very difficult to use after childbirth and miscarriage, and when menopause is approaching, because cycle lengths can be very irregular during these times.
       Despite all of these difficulties, users of the "rhythm" method experience a user effectiveness rate of 91% during the first year, which is far better than most mechanical methods of contraception.[3] The success rate is even better when the method is combined with temperature observations.

       The Basal Body Temperature (BBT) Method of NFP. The basal body temperature is the temperature of your body at rest, when it is unaffected by activity, food or drink. A woman who takes her temperature with a basal thermometer at about the same time each morning will find there is a definite monthly pattern to her temperatures. She will often also find that her BBT dips before ovulation and rises thereafter until menstruation. The BBT is generally used only to determine the beginning of postovulation infertility.

       The Ovulation Method of NFP. This method of NFP is sometimes called the cervical mucus charting method. There are several variations of the ovulation method, including the Billings Ovulation Method (BOM) and Dr. Thomas Hilgers' Creighton Model.

       The ovulation method is based upon the regular pattern of changes in the cervical mucus during the menstrual cycle. The quantity and quality of this mucus in terms of slipperiness, stretchiness, wetness and tackiness will change from day to day as a woman approaches ovulation. Ovulation usually occurs within one day before or after the last day of the most slippery or fertile mucus, i.e., the "Peak Day." Generally, users of the ovulation method start noting the fertile period starting with the appearance of any external mucus after menstruation has ceased. They regard the time of postovulation infertility as starting on the fourth day after the Peak Day.

       Sympto-Thermal Method (STM). The sympto-thermal method combines observations of basal body temperature and cervical mucus, and, as an optional cross-check, adds an examination of the cervical os (mouth of the cervix) as well. During fertile times, the osopens, the cervix rises, and its tip becomes softer. Some women also experience regular episodes of mittelschmerz, or pain associated with ovulation. During infertile times, theos closes, the cervix descends, and the tip becomes firmer.
       The end of pre-ovulation infertility is determined in several different ways. As a general rule, couples may resume intercourse on the fourth day following the "Peak Day" of mucus and the third day of upward thermal shift. As with other methods of natural family planning, the period of abstinence may be relatively long for the first few cycles of practice, after childbirth or miscarriage, or after switching from an abortifacient method involving hormones. The average experienced couple using the sympto-thermal method has about 9 or 10 days of abstinence each cycle. Every time that a fertile type of mucus appears before ovulation, they must abstain for three days. Once ovulation occurs, the couple are sterile until menstruation and usually sterile during the first two days of menstruation.

       Refinements of Natural Family Planning. Today there are a number of devices available to help couples estimate the day of ovulation. The most practical and effective of these is the Japanese "L-Sophia."
       Several other approaches to improving the effectiveness of NFP are under study. One device would measure hormone levels in urine and would therefore be a true ovulation predictor or detector. Another would accurately measure the water level in cervical mucus. Others would detect preovulatory rises of estrogen in saliva and cervical mucus. In addition, changes in breast milk and in the electrical resistance of cervical mucus are under investigation.
       One conclusion is certain: Thirty years of contraceptive and abortifacient research have shown that there is no way to subdue our fertility without paying a steep price, both on the individual and societal level. The only way to be in harmony with our fertility is to let it take its natural place in our lives as a friend to cooperate with, not as an enemy to be defeated, subdued or snuffed out.

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Endnotes for “Fertilization Age and Gestational Age”

[2] Since the early 1980s, there has been discussion regarding the best term that could be applied to the methods used by couples who want to place their reproductive lives in the hands of God. Many pro-lifers object to the term "natural family planning," saying that it smacks of utilitarian Planned Parenthood-type thinking. They prefer instead "natural fertility awareness" (NFA) or "natural fertility regulation" (NFR). NFA refers to the teaching of signs and symptoms of fertility to mature teens in preparation for marriage, and is thus different from NFP. NFR is a term that is certainly appropriate for people who have been using the methods for some time and are therefore knowledgeable enough to avoid the confusion brought on by the use of several different terms for the natural methods.
       However, the term "natural family planning" (NFP) has two distinct advantages. First, it appeals strongly to couples using contraception and considering changing to the natural methods, but who still possess lingering traces of the "contraceptive mentality." Pro-lifers should always try to wean their friends and acquaintances away from abortifacients and contraceptives. Secondly, the term "NFP" is used by many teachers in developing countries to draw a sharp contrast between natural methods of fertility regulation and contraceptive/abortifacient methods. These teachers find that their students, who often have little education, are confused by other terms.
       For these reasons, the term "natural family planning" (NFP) is used throughout this chapter. This is no way implies that other terms are less appropriate.

[3] Robert A. Hatcher, et. al. Contraceptive Technology (17th Revised Edition) [New York: Ardent Media, Inc., 1998]. Table 31-1, "Percentage of Women Experiencing an Unintended Pregnancy During the First Year of Typical Use and the First Year of Perfect Use of Contraception and the Percentage Continuing Use at the End of the First Year: United States," page 800.