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303, February 2014 Society of Obstetricians and Gynaecologists of Canada.
Multifetal gestations: twin, triplet, and higher-order multifetal pregnancies.
Page JM, The risk of stillbirth and infant death by each additional week of expectant management in twin pregnancies.
Risk of late-preterm stillbirth and neonatal morbidity for monochorionic and dichorionic twins.
Determining the Estimated Due Date The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period. The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle. 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.