Accurate assessment of gestational age is of paramount importance for management of pregnancy, interpretation of test results, and timing of interventions. Pregnancy dating should be determined by a combination of the historical, clinical, and laboratory criteria listed below:
- known normal last menstrual period (LMP), corrected for cycle length
- date(s) of HCG testing (urine or blood)
- uterine size on initial physical exam
- date of detection of FHT with Doppler
- first trimester ultrasound measurement of crown-rump length
- second trimester ultrasound measurement of multiple fetal biometrics
If the pregnancy resulted from assisted reproductive technologic (ART) intervention, then the ART-derived gestational age should be used to assign the EDD.
Ultrasound dating should take precedence over clinical dating if:
- the LMP is uncertain or abnormal,
- there is a more than 5 day discrepancy between historical / clinical parameters and ultrasound measurements at ≤ 8⁶⁄₇ weeks, or there is a more than 7 day discrepancy between historical/clinical parameters and ultrasound measurements at 9⁰⁄₇ – 15⁶⁄₇ weeks, or
- there is a more than 10 day discrepancy between historical / clinical parameters and ultrasound measurements at 16⁰⁄₇ – 21⁶⁄₇ weeks, or
- there is a more than 14 day discrepancy between historical / clinical parameters and ultrasound measurements at 22⁰⁄₇ – 27⁶⁄₇ weeks. 1-2
Gestational age based on third trimester ultrasound should be interpreted with caution, due to the decreased accuracy of ultrasound for dating late in pregnancy.
When necessary for clinical decision making, fetal pulmonary maturity may be presumed if early reliable dating confirms that the patient is no more than seven days before the estimated date of delivery (EDD).
As soon as data from the LMP or first accurate ultrasound (or both) are obtained, the EDD should be determined, documented in the record, and discussed with the patient by the primary obstetrical provider. Use of electronic health record functionality or ACOG EDD calculator to confirm EDD is recommended.
- Ultrasound in pregnancy. ACOG Practice Bulletin No. 175. December 2016. Reaffirmed 2020. American College of Obstetricians and Gynecologists.
- Methods for estimating the due date. ACOG Committee Opinion No. 700. May 2017. American College of Obstetricians and Gynecologists.
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