The tendency for multiple gestations to be delivered earlier than singleton pregnancies should not be interpreted that multiples should be assigned an earlier estimated due date.
American College of Obstetricians and Gynecologists.
Timing of sexual intercourse in relation to ovulation.
303, February 2014 Society of Obstetricians and Gynaecologists of Canada.
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).
The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.