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“Weeks from fertilization” is the method used by CBR to age aborted embryos and fetuses. The reason for our use of this aging convention can be found in an article published in The Orange County Register newspaper (Associated Press, April 17, 2008) headlined “Study: Gender, other factors guide preemies’ survival.” The article reported a study published in the same week’s issue of the New England Journal of Medicine. The subject of the study was an analysis of refined methods for determining which types of care are most likely to improve survival rates among prematurely born infants. For the purposes of the study, the babies involved were aged using what CBR calls “gestational age” (sometimes called “fetal weeks”) which is to say, “weeks from fertilization,” as opposed to “menstrual weeks,” which are measured from the first day of the mother’s last menstrual period – LMP. Using “weeks from fertilization”, pregnancy lasts 38 weeks. Using “menstrual weeks,” pregnancy lasts 40 weeks.
This study offers still more evidence that when research scientists conduct clinical studies which require that a baby’s age be measured with as much precision as possible, they don’t use “menstrual weeks” to calculate the baby’s age, they use “weeks from fertilization.” The latter term is based on objective anatomical measurements and other developmental observations instead of the basis for “menstrual week” aging which is based on a mother’s ability to recall her a menstrual history she has usually not been documenting. The resulting uncertainty is compounded by the fact that her reproductive cycles might be highly variable and involve mere “guess work” assumptions as to when fertilization actually occurred.
Precision is why CBR uses the “weeks from fertilization” standard. The term “gestational age” is sometimes used ambiguously to also refer to “menstrual weeks” but we use it only to describe “weeks from fertilization” because the word “gestation” refers to the baby’s growth and the term “menstruation” refers to the mother’s reproductive cycle. Embryologists tend to age the embryo and fetus in “weeks from fertilization” because their focus is on the baby. Obstetrician/Gynecologists usually time pregnancy in “menstrual weeks” because their focus is on the mother.