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The American College of Obstetricians and Gynecologists.
Berghella V, et al., Does transvaginal sonographic measurement of cervical length before 14 weeks predict preterm delivery in high-risk pregnancies?
The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.
"Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" .
Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7].
Otherwise "..avoid missing a situation of early intrauterine growth restriction in one twin, most experts agree that the clinician may consider dating pregnancy using the larger fetus." [22,23].
Observed Gestational Age at Delivery The table below shows the average age of infants delivered in the United States according to the number of fetuses being carried (plurality) . Savitz DA, Terry Jr JW, Dole N, et al: Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination.
Determining the Date of Conception Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception.
However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age and, therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.