Historically, dating pregnancies and calculating due dates were left to weekly pregnancy calendars. However, ultrasound dating, in particular first trimester sonography, has greatly improved our ability to calculate the estimated due date (EDD). There will be times that dating based on LMP does not match the ultrasound date.
ACOG recommends redating as follows:
Clinical determination of EDD, 280 days after the last menstrual period (LMP) still plays a role but may not always be accurate due to variability in length of an individual woman’s cycle length or timing of ovulation. Accurate dating is vital to pregnancy management, as certain interventions and management decisions may be based on such information including timing of delivery in the case of pregnancy complications.
ACOG/AIUM/SMFM Committee Opinion 700: Methods for Estimating Due Date
ACOG AIUM Practice Bulletin 175: Ultrasound in Pregnancy
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