It is appropriate to change the gestational age or EDD when the data used for screening are substantially above or below that determined by ultrasound. Recalculating for incorrect gestational age is not recommended when the screen is positive for trisomy 18 (see Question 4).
The earliest EDD calculated by ultrasound should be used for dating purposes.2 An ultrasound derived EDD is most accurate when determined in the first trimester. Accuracy decreases with advancing gestational age. For example, accuracy of an ultrasound EDD is ±7 days in the first trimester and ±10 days in the second trimester.
If a first trimester ultrasound EDD is available and the gestational age used for screening is within the EDD ±7 days, the gestational age should not be changed for screening purposes. Similarly, if second trimester (but not first trimester) EDD is available and the gestational age used for screening is within the EDD ±10 day range, the gestational age should not be changed for screening purposes. If the gestational age used for screening is outside the ultrasound EDD range, it may be appropriate to change the gestational age used for screening.
If you want to change the EDD/gestational age used for a specific patient’s screening test, please contact your local Quest Diagnostics laboratory or call Quest Genomics Client Services at 866.GENE.INFO. If the revised gestational age is between 15.0 to 22.9 weeks gestation, we can calculate and report new risks. If the revised gestational age is ≥14.0 weeks and <15.0 weeks, we can calculate and report new Down syndrome and trisomy 18 risks, but not ONTD risks (see Question 7). If the revised gestational age is <14.0 weeks, we cannot calculate a new risk. Consider submitting a second specimen for screening, collected when the woman is between 15.0 to 22.9 weeks gestation (preferably 16 to 18 weeks). If the revised gestational age is >22.9 weeks gestation, we cannot calculate a new risk, and a more accurate risk assessment cannot be provided.