A simple clinical formula for predicting fetal weight in labour at term--derivation and validation

S Afr Med J. 2009 Jun;99(6):457-60.


Objectives: To derive and validate a simple formula for birth weight based on symphysis-fundal height (SFH) measurement during labour, and to determine a useful SFH cut-off value for prediction of birth weight > or = 4000 g.

Methods: In a derivation study, SFH was measured in women at term in the active phase of labour. A simplified formula for birth weight was derived from a regression equation. The best cut-off SFH measurement was obtained for prediction of birth weight > or = 4000 g. After this, a similar study was done to validate these findings.

Results: In the derivation study (N = 504), birth weight was predicted by the equation: birth weight in g = 301 + 78 (SFH in cm). This was transformed to the simplified formula: birth weight in g = 100 ([SFH in cm]-5). Using this formula for the data set, 68.1% of birth weight estimates were correct to within 10% of the birth weight. For prediction of birth weight > or = 4000 g, an SFH measurement of 40 cm had a sensitivity of 82% and a specificity of 80%. In the validation study (N = 294), the derived simplified formula gave 65.0% of estimates correct to within 10% of the birth weight. The predictive values of the 40 cm SFH cut-off were similar to those in the derivation study.

Conclusion: The derived simplified formula was validated in the second study. The formula may be useful for intrapartum use in term pregnancies. A cut-off SFH measurement of 40 cm may identify labours at risk for cephalopelvic disproportion or shoulder dystocia.

Publication types

  • Research Support, Non-U.S. Gov't
  • Validation Study

MeSH terms

  • Adult
  • Birth Weight*
  • Body Mass Index
  • Female
  • Fetal Weight*
  • Humans
  • Infant, Newborn
  • Models, Statistical
  • Predictive Value of Tests
  • Pregnancy
  • Term Birth*