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APPENDIX B

OBSTETRIC DATES AND INTERVALS

MONTH PRENATAL CARE BEGAN

In 1988, the Vermont Department of Health adopted the National Center for Health Statistics method of calculating the month prenatal care began. This allows Vermont data to be more easily compared to national data, and eliminates any errors that could be introduced by hospitals using different reporting methods.

The calculation is based on the month of last menses and the month of the first prenatal visit. For example, if the date of last menses was January 1 and the date of first prenatal visit was March 10, the calculation shows that prenatal care began in the second month.

GESTATIONAL AGE

Live Births and Fetal Deaths: National Center for Health Statistics methodology is used to determine gestational age. The weeks of gestation are computed from the date of last menses and the date of birth, if the data are available and the computed weeks are between 15 and 55.

An edit is then run that checks whether the birth weight of the record is within two standard deviations of the average birth weight for the calculated weeks of gestation. If the birth weight is within the bounds for the calculated weeks of gestation, then the calculated weeks of gestation becomes the assigned gestational age. If the birth weight is not within the bounds for the calculated weeks, but it is within the bounds for the clinical estimate of gestation, then the clinical estimate of gestation is assigned as the gestational age. If the birth weight does not fit within the bounds for either the calculated weeks of gestation or the clinical estimate of gestation, the gestational age is designated "unknown". If the month and year (for date of last normal menses) are given, but the day is missing, the gestational age is imputed using the gestational age of the preceding record that has complete data with the same computed month of gestation and the same 500-gram birth weight interval.

In 2000, 88.1 percent of the records used the calculated weeks, 10.6 percent used the clinical estimate of the weeks, and the remaining records were designated "unknown" gestational ages.

Abortions: Gestational age is calculated from the date of last normal menses and date of abortion. The clinically estimated weeks is used if the date of last normal menses is unknown, or if the calculated weeks and the clinically estimated weeks are more than two weeks apart.

NUMBER OF PRENATAL VISITS

Since 1985, hospitals report the date of the first and last recorded prenatal visits and the number of visits recorded. In 1986, a procedure was adopted for births of 36 or more weeks of gestation with less than 20 reported visits, and whose last reported visit was within four weeks of delivery. In these cases the Vermont Department of Health records the number of prenatal visits reported by the hospitals plus one visit for every week between the date of the last recorded visit and the delivery date.

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