Diagnosis of small-for-gestational-age fetuses between 24 and 32 weeks, based on standard sonographic measurements

To create and validate a formula using sonographic biometry measurements for the optimal diagnosis of small-for-gestational-age (SGA) fetuses between 24 and 32 weeks of gestation.

A logistic model using gestational age, femur diaphysis length, abdominal and head circumferences to diagnose SGA was set up in a first group of 64 fetuses born between 24 and 32 weeks (group I). A Receiver Operating Characteristic (ROC) curve was drawn. Our model was compared with standard single ultrasound measurements or combined into an estimated fetal weight (EFW) formula. An external validation was carried out on a second group of 183 fetuses (group II) from another maternity unit (ROC curve and comparisons).

The area under the ROC curve was 0.91 in group I and 0.93 in group II. Using a 0.5 cut off point for our model yielded a sensitivity of 76% and specificity of 91% for group I. This model is more specific than most other measurement methods with a similar sensitivity. Using the same cut off point (0.5) in Group II, our model was more specific (98%) but less sensitive (66%) when compared with single ultrasound measurements and EFW formulae. By varying the cut off point, we were able to demonstrate that, for a similar sensitivity, our model had a higher specificity than single ultrasound measurements and had similar specificity to EFW formulae.

The logistic model we set up was able to calculate an SGA risk score between 24 and 32 weeks of gestation in a population at high risk for elective delivery. The cut off point with a view to diagnosis can vary and makes it possible to give greater importance to the sensitivity or specificity depending on the clinical context.