Pregnancy Gestation by LMP and Ultrasound Biometry

Predicts estimated delivery dates and gestational age using LMP and biometry

The global unit selector only affects unanswered questions
1.Current date?
2.1st day of last menstrual period?
3.1st Trimester measurement crown rump length?
4.2nd Trimester biparietal diameter?
5.2nd Trimester head circumference?
6.Ultrasound date?
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1. Current date?

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About this Calculator

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.

Ultrasonic fetal measurements: new Australian standards for the new millennium.

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

Altman DG, Chitty LS.

New charts for ultrasound dating of pregnancy.

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

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