Timing of sexual intercourse in relation to ovulation.
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.
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).
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].
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.