Dating the Beginning of Pregnancy

One of the most important aspects of obstetrical care is to date, as precisely as possible, the beginning of pregnancy so that the estimated time of arrival (ETA) can be calculated (sometimes referred to as the estimated date of confinement – EDC).  The standard textbook of obstetrics, Williams Obstetrics, states vigorously that “precise knowledge of the age of the fetus is imperative for ideal obstetrical management!” (emphasis in the original).  And yet, even with all of the available technology, one of the puzzles of modern obstetrics, is that the obstetrician has not yet learned how to accurately date the beginning of a pregnancy.

Pregnancy can be measured in two different ways.  The most common and most often used in clinical obstetrics is the measurement of the gestational age of the pregnancy.  The gestational age of the pregnancy is measured from the first day of the last menstrual period.  In this way of dating the pregnancy, the pregnancy is 40 weeks in duration (on average) instead of the actual 38 weeks.  In other words, it dates the pregnancy, on average, two weeks longer than it is.

The other way of measuring the dates of the pregnancy is to measure the fetal age.  The fetal age of the pregnancy is measured from the time of conception or the estimated time of conception (ETC).  When measuring the pregnancy in this fashion, it will be 38 weeks long or two weeks shorter than the gestational age dates.  The fetal age, of course, is the actual age of the pregnancy.

Historically, the obstetrician has focused on the first day of the last menstrual period for two reasons.  First of all, the menstrual flow itself is a fairly dramatic symptom which the woman can be expected to remember.  In addition, it is easy to teach her to record the first day of the last menstrual period so that when that information is elicited by the physician, at a later time, it is available.

However, in the midst of all of this, the obstetrician and many women have missed the point that the cervical mucus discharge is very much a flow in the same fashion as the menstrual flow.  In some countries, they refer to menstruation as the red flow and the mucus discharge as the white flow.  Unfortunately, modern obstetrics has paid little attention to the white flow.

When one is charting the Creighton Model FertilityCare™ System (CrMS), however, one can date the pregnancy accurately from the actual or estimated time of conception.  Therefore, one can date the pregnancy according to its true date (or true beginning) or in fetal age terms.  This is measured by evaluating the acts of intercourse that occur during the time of fertility and establishing an estimated time of conception through this approach.

We have studied 173 patients in a consecutive fashion who have been charting the CrMS at the time of conception.  The ETC and the ETA were calculated.  In this group of patients, early ultrasound dating of the pregnancy was also obtained.  In Table 53-3, the results of that evaluation are shown.  An accurate date for the beginning of pregnancy can be established with the use of the CrMS.  This correlates well with the ultrasound dates and the estimated due date or ETA.  In fact, the CrMS dating correlated, on average, in the following way:  CrMS ETA = U/S, ETA + 1.97 days.  The CrMS dates were highly accurate with the two being within 10 days of each other in 100 percent of the cases.

An example of this dating is shown in Figure 53-3.  Here, a pregnancy occurring in a longer cycle where the Peak Day occurred on Day 26 of the cycle is shown.  If this pregnancy is calculated in the traditional fashion, there will be a 12-day discrepancy between the dates as customarily calculated from the last menstrual period and the actual date of the pregnancy which is calculated from the Peak Day.  The Peak Day, of course, will be much more accurate.

Figure 53-3, p. 710 from the NaPro textbook. A pregnancy occurring in a longer cycle where the Peak Day occurred on day 26 of the cycle (From: Saint Paul VI Institute research, 2004).