Pregnancy Dating

Step 1: Document First day of the last normal menstrual period then use Nigel's rule or use uptodate EDD calculator to estimate EDD

• If pregnancy resulted from assisted reproductive technology (ART), the ART-derived gestational age should be used to assign the estimated due date (EDD). For instance, the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo and the date of transfer.

Step 2: Compare with EDD from first TM ultrasound. Look at Table 1 to see if we should use EDD based on LMP or US

• Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.

Step 3: Determined EDD

• As soon as data from the last menstrual period (LMP), the first accurate ultrasound examination, or both are obtained, the gestational age and the EDD should be determined, discussed with the patient, and documented clearly in the medical record.
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• A pregnancy without an ultrasound examination that confirms or revises the EDD before 22 0/7 weeks of gestational age should be considered suboptimally dated.

Estimating due date