[Comparison of 2 indexes of prenatal care and risk of preterm delivery]

A new index the adequacy of prenatal care utilization (APNCU) index, has been proposed to provide a more accurate and comprehensive measure of antenatal care use than the widely used Kessner index. To better understand the value of the two above mentioned indexes as predictors of preterm delivery, we examined their ability to predict women who will or will not deliver before 37 weeks of gestation.

A case-control study was performed, including 207 cases and 381 controls. Prenatal care was assessed on the basis of the two above mentioned indexes, both taking into account the number of prenatal care visits, the date of the first visit and gestational age. Multiple-factor adjusted odds ratios and their 95% confidence intervals were estimated using logistic regression methods.

The Kessner index showed a lineal trend with both crude and adjusted for (the APNCU index) estimates of preterm delivery risk. The APNCU index did not show any linear trend with adjusted for (the Kessner index) estimates of preterm delivery risk. To assess whether the Kessner index added explanatory information to the APNCU index (or vice versa), the APNCU index was regressed on the Kessner index (and viceversa), and a set of residuals was computed for both indexes. In logistic regression analyses, the residuals of Kessner added meaningful information to the APNCU index, whereas the residuals of the APNCU index did not add any relevant information to the Kessner index. There results remained unchanged after controlling for several confounders.

A variation in the definition of adequate prenatal care use changes the association between prenatal care and preterm delivery. The Kessner index showed a better ability for discriminating and predicting the risk of preterm delivery.