Managing Labor and Delivery of the Diabetic Mother

Hen Y. Sela, MD; Itamar Raz, MD; Uriel Elchalal, MD


Expert Rev of Obstet Gynecol. 2009;4(5):547-554. 

In This Article

Estimating Fetal Weight

Knowledge of expected birthweight is important for obstetricians, as it is an important variable affecting perinatal mortality.[72] Fetal weight estimation is thought to be helpful in managing the delivery of a large baby, where complications may occur.[73] The two most widely available means of estimating fetal weight-clinical assessment and ultrasound-have been shown to have roughly equivalent accuracy, even in macrosomic fetuses,[74,75,76] making it difficult to recommend one method over the other based on hard evidence. Nonetheless, obtaining a fetal weight estimate by ultrasound provides some measure of objectivity over clinical estimation and has been shown to be as accurate in obese women as in lean women.[77]

The two main ultrasonic methods used for predicting a macrosomic fetus are based on measurement of fetal AC and EFW.[78] It is currently unclear which of these two ultrasound methods has better diagnostic accuracy in predicting macrosomia. A recent systemic review aimed at evaluating the accuracy of biometry in prediction of macrosomia demonstrated the following:[79]

  • The most commonly used formulae for estimating fetal weight were Hadlock's formula, using femur length and AC,[80,81] and Shepard's formula, using biparietal diameter and AC measurements;[82]

  • The pooled positive likelihood ratio for an ultrasound EFW of more than 4000 g to predict an actual birthweight of more than 4000 g was 5.7 (4.3-7.6) by Hadlock's formula (femur length/AC)[78,79,80] with a respective negative likelihood ratio of 0.48 (0.38-0.60);

In total, 12 studies evaluated the accuracy of AC to predict a birthweight of more than 4000 g, 4500 g or higher than the 90th percentile for gestation. The pooled LRs for positive and negative tests of an AC of more than 36 cm to predict a birthweight of more than 4000 g were 6.9 (5.2-9.0) and 0.37 (0.30-0.45), respectively. The corresponding likelihood ratios for an AC over the 90th percentile to predict a birthweight of higher than the 90th percentile for gestation were 4.2 (2.3-7.7) and 0.33 (0.21-0.54), respectively.

Generally, a positive test result was found to be more accurate for ruling in macrosomia than a negative test result for ruling it out.

MRI and 3D ultrasound are promising new modalities that may improve fetal weight estimation by providing volumetric assessments of the fetus.

In general, both 3D ultrasound and MRI result in more accurate fetal weight estimates than 2D ultrasound. Most of these studies are limited in their applicability to the issue of fetal weight estimation in diabetic women for several reasons: overall sample sizes are small, few include or have much less focus on a diabetic population and fetuses at the extremes of weight are few in number. In addition, the performance of these modalities in routine clinical use (i.e., outside of a research setting) has not been evaluated and their availability and cost are also potential obstacles.[83,84,85,86,87]


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