The "Adequacy of Prenatal Care Utilization" (APNCU) index to study low birth weight: is the index biased?

A recent, nationwide study of 54 million births reported increasing trends toward more prenatal resource utilization from 1981 to 1995, when other indicators have shown worsening trends in birth outcomes. The Adequacy of Prenatal Care Utilization (APNCU) Index was used to measure resource utilization, but the Index appears to be biased because women grouped in the intensive category have the highest rates of low birth weight (LBW). The objective of this paper is to provide a systematic examination of the Index and to uncover biases that may preclude its use in analyzing the association between resource utilization and birth outcomes. This is a cross-sectional study including all singleton live births in 1993 through 1996 (n = 591,403) in Ohio. Birth certificate data are used to derive the Index, which categorizes women as follows: Adequate Plus (A+), Adequate, Intermediate, and Inadequate. The Index is based on the ratio of observed to expected (O/E) number of prenatal visits. The expected number of visits is based on the American College of Obstetricians and Gynecologists (ACOG) recommendations. The Index also considers the month of initiation of prenatal care. The outcome measures are low birth weight (LBW) and small-for-gestational age (SGA). The LBW rate is 11.8% in the (A+) category, compared to 9.4% in the Inadequate category, and 3.3% and 3.5% in each of the Intermediate and Adequate categories, respectively. Preterm births are disproportionately represented in the (A+) category: 61.2% of births prior to 37 weeks are (A+), whereas only 18.9% of term births are (A+). This apparent bias results from the fact that the ACOG schedule of prenatal visits allocates nearly one third of the total visits to the last 4-5 weeks of gestation. A shorter gestational age implies fewer number of expected visits, a smaller denominator in the O/E ratio, and O/E ratios exceeding 100% by large margins. In fact, the observed number of visits exceeds the expected number of visits by only one or two in 40.1% of all births grouped in the (A+) category. Consequently, the Index yields misleading results indicating that women grouped in the (A+) category (or O/E ratios > 110%) are most likely to deliver LBW infants. Contrary to the results obtained through the APNCU Index, our gestational age-specific analysis showed that increasing number of prenatal visits is associated with improved birth outcomes. We recommend that the use of the APNCU Index to study the association between prenatal resource utilization and LBW be discontinued.