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Prenatal Care Calculation Revised

Overview of the Change

Please note that changes have been made to the calculation used to determine the trimester of each mother's first prenatal care visit, which are detailed below. The changes have been implemented, retroactively, back to 2003 in EDDIE, in the annual birth statistics reports, and in the Healthy People reports. Starting with 2009 birth data, all occurrences of prenatal care statistics are calculated using the revised methodology.

Reasoning Behind the Change

Beginning with the reporting of 2003 live births, Pennsylvania implemented the latest revision of the U.S. Standard Certificate of Live Birth. Prior to 2003, the Certificate of Live Birth collected the month prenatal care began. Beginning in 2003 the Certificate of Live Birth was revised to collect the exact dates (month/day/year) of the first and the last prenatal visits. Despite this change, the calculation and reporting for trimester of the first prenatal care visit did NOT change. This was done in order to make trending our (Pennsylvania) data possible with previous years. However, since there are now adequate data available using the revised Certificate of Live Birth, the trimester of first prenatal care visit calculation is being updated to utilize the last normal menses date and the date of the first prenatal care visit. As a result, the newly implemented methodology is in line with the National Center for Health Statistics methods for determining the trimester of the first prenatal care visit, allowing for Pennsylvania vs. U.S. comparisons.

A Closer Look at the Calculation

The revised methodology calculates trimesters differently than the previous method. The previous calculation was limited by what was collected on the previous version of the Certificate of Live Birth. This meant that the previous calculation only determined the number of months between the last normal menses and the month of the first prenatal care visit. With the revised methodology, the complete dates for the first prenatal care visit and the last normal menses are used in the calculation in order to provide a more accurate determination of the trimester of the first prenatal care visit. The trimester is determined, utilizing the following definition from the National Center for Health Statistics: 1st Trimester is less than 92 days elapsed; 2nd Trimester is between 92 and 182 days elapsed; and 3rd Trimester is between 183 and 304 days elapsed. If the number of days elapsed is more than 304 days, the trimester of first prenatal care visit is categorized as unknown. Additionally, if the month of the last normal menses is unknown and the obstetric estimate of gestation is known, then the first day of the last normal menses is estimated to be the date of birth minus the obstetric estimate of gestation. Again, the aforementioned changes to the calculation will allow for a more accurate look at when the first prenatal visit took place.

Affects of the Change

The result of the change in calculation is a large decrease in births that fall into the unknown category for trimester of the first prenatal care visit. As determined by the revised calculation, many of the previously unknown trimester births have transferred to the second and third trimesters. This not only affects the distribution counts in each category, but it also affects the percentages since the number of births with an unknown trimester of first prenatal care visit are removed from all calculations. Based on the revised calculation, the increase in the number of records in the second and third trimesters taken from the unknown category resulted in a decrease in the percentage of births receiving care in the first trimester. In other words, to determine the percentage each trimester accounts for, you take the total number for the trimester (this is the numerator), and divide by the total number of births minus the unknown trimester total (this is the denominator). As stated previously, the total number of unknown trimester births decreased, which means there was less to remove from the denominator. Below is a breakdown of the impact made using the revised methodology, looking at 2008 birth data:

Resident Live Births by Trimester of First Prenatal Visit, Pennsylvania, 2008
Previous CalculationRevised Calculation
Total Births148,934100.0148,934100.0
First Trimester98,65779.497,22470.5
Second Trimester19,43215.631,07922.6
Third Trimester4,2933.57,6095.5
No Prenatal Visits1,9091.51,9091.4