In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. American College of Obstetricians and Gynecologists. Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period). Time of implantation of the conceptus and loss of pregnancy. Daily immunoactive and bioactive human chorionic gonadotropin profiles in periimplantation urine samples. American College of Obstetricians and Gynecologists.
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]. Number 4, May 1999 (replaces educational bulletin Number 147, October 1990). Number 9, October 1999 (replaces Technical Bulletin Number 188, January 1994). 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 gestational age would, therefore, be at least 3 to 4 weeks at the time of a reliable h CG pregnancy test. Twins When a twin pregnancy is the result of in vitro fertilization determination of gestational age should be made from the date of embryo transfer. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997). 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" . Other Methods For Estimating the Gestational Age Clinical Examination A pelvic examination supported by good menstrual records in the first trimester has been reported to be a reliable method for dating of pregnancy . Doppler Ultrasonography The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients . Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH.