Dating the Beginning of Pregnancy



A pregnancy occurring in a longer cycle where the Peak Day occurred on day 26 of the cycle.

One of the most important aspects of obstetrical care is the calculation of the estimated time of arrival (ETA) during early pregnancy. Because multiple assessment procedures and treatment options are dependent on the gestational age or fetal age, determining the beginning of a pregnancy is especially important during a complicated pregnancy. Fetal age and gestational age are the two alternative ways to describe the estimated duration of a pregnancy.

Although the fetal age and gestational age of a pregnancy are never identical, they describe the same progression of pregnancy. Gestational age is the most common and traditional way. From the first day of the woman's last menstrual cycle, conception is assumed to have occurred some time around the second or third week. The pregnancy is expected to continue until 40 weeks from the start of the last menstrual cycle. In contrast, fetal age begins at the estimated time of conception (ETC). A fetus with a gestational age of 12 weeks will have a fetal age of 10 weeks. The expected time of arrival for a pregnancy is 40 weeks gestational age, or 38 weeks fetal age.

An often-overlooked way to calculate the fetal age is charting the Creighton Model FertilityCare System (CrMs) from cervical mucus discharge flow and acts of intercourse. Cervical mucus flow indicates a time of fertility. Intercourse during a time of fertility can therefor be evaluated to establish the estimated time of conception. By studying 173 patients who charted the CrMs time of conception, we have determined that CrMs is an acceptable method to determine fetal age. Ultrasound (U/S) determined estimated due date and CrMs estimated due date correlated, on average, in the following way:  CrMS ETA = U/S, ETA + 1.97 days. CrMs ETA and Ultrasound were within 10 days in 100 percent of the 173 studied cases.

The CrMs determination of the fetal age is more accurate than the menstrual cycle determination of gestational age, despite the 12-day discrepancy between the two methods