It is very important that the dating of the pregnancy, finding the right due date, is accurate because most decisions in pregnancy relate to having the right due date.

Calculating the estimated due date (EDD) is among the most important results of evaluation and history taking early in prenatal care. The EDD is important for the timing of many different aspects of obstetric care such as scheduling and interpretation of certain antepartum tests, determining the appropriateness of fetal growth, and designing interventions to prevent preterm births, post-term births, and related morbidities. 

Recommendations

1. If there is a well established day of ovulation (which is also the day of fertilization) either by basal body temperature or by ovulation predictor kit (OPK) then this should be used to determine the due date (add 266 days to the ovulation day)
2. If the pregnancy is the result of assisted reproductive technology (ART) and in-vitro fertilization (IVF), the clinician should use the ART-derived gestational age to assign the EDD. For example, for a pregnancy that results from in vitro fertilization, the clinician should use the age of the embryo and the date of the transfer to establish the EDD.
3. High-quality ultrasound measurement of the embryo or fetus during the first trimester of pregnancy (before 14 weeks) is the most accurate method of establishing or confirming gestational age.
4. As soon as the clinician has data from the last menstrual period, the first accurate ultrasound examination, or both, the gestational age and the EDD should be calculated, discussed with the patient, and recorded clearly in the patient's medical record.
5. The clinician should use the best obstetric estimate, rather than calculations based only on the last menstrual period, to determine gestational age.
6. Subsequent changes to the EDD and redating the pregnancy should only be made in rare circumstances, should be discussed with the patient, and should be recorded clearly in the patient's medical record.