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SAMJ: South African Medical Journal

Print version ISSN 0256-9574


BUCHMANN, Eckhart  and  TLALE, Karabo. A simple clinical formula for predicting fetal weight in labour at term: derivation and validation. SAMJ, S. Afr. med. j. [online]. 2009, vol.99, n.6, pp. 457-460. ISSN 0256-9574.

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 >4 000 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 >4 000 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 >4 000 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.

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