Pregnancy Gestation by LMP and Ultrasound Biometry

Predicts estimated delivery dates and gestational age using LMP and biometry


Pregnancy dating is conducted using the best available information at the appropriate timeline during gestation. Early in the first trimester, the last menstrual period (LMP) is the historical data used to estimate the gestational age and delivery date. A dating ultrasound performed in the first trimester uses the anatomic parameter “crown rump length” to estimate the gestational age (not meant to be used outside of the 1st trimester). During the second trimester, dating can be achieved through head circumference (HC) and “biparietal diameter” (BPD) - two other fetal biometric parameters. As noted by the authors in the derivation studies1,2 these two parameters do not perform as reliably in the 3rd trimester and thus should be used with caution during that window of time. All predictions for delivery dates are estimates that rely on relatively few objective parameters - they do not account for the dynamic physiologic changes that occur throughout pregnancy and influence the actual delivery date.

Variable & Associated Points

  • Gestation by LMP is calculated by the total difference in days between entered current date and entered last menstrual period date
  • Gestation by CRL in Weeks = 5.2876 + (0.1584 * CRL) - (0.0007 * (CRL^2))
  • Gestation by HC in Weeks = e^ (1.854 + [(0.010451 * HC) - (0.000029919 * HC^2) + 0.000000043156 * HC^3)]
  • Gestation by BPD in Weeks = (2 * BPD (mm) + 44.2) / 7


Westerway SC, Davison A, Cowell S.

Aust N Z J Obstet Gynaecol. 2000 Aug; 40(3):297-302.

Altman DG, Chitty LS.

Ultrasound Obstet Gynecol. 1997 Sep;10(3):174-91.

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