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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: The methodology used to determine gestational age was changed this year to better match the methodology used by the National Center for Health Statistics.

        Two measures of gestational age are available from the standard certificates. The standard certificates collect information on the first day of the mother's last menstrual period (LMP). The interval between the LMP and the date of birth is the calculated gestational age. The standard certificate also collects a clinical estimate of gestation.

        The primary measure used by NCHS to determine the gestational age of the newborn is the calculated weeks of gestation. However this method is subject to error due to imperfect maternal recall and to misidentification of the LMP. Therefore an edit is run that checks whether the calculated weeks of gestation is consistent with the infant's plurality and birth weight. 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 LMP 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.

        Vermont's methodology was different in two respects. (1) The birth weight edit used in Vermont was more stringent than that used by NCHS. As a result approximately 2.5% fewer records each year used the calculated weeks gestation. For most of these records the clinical estimate was used. (2) The method used by NCHS to impute the gestational age could not be replicated in Vermont since the selection of the imputed value was dependent on the order of records in the file being used. In these cases in Vermont the clinical estimate was used as long as it met the birth weight edit.

        An analysis was undertaken this past year to compare the results of these two slightly different methodologies. An unanticipated finding was that the Vermont method resulted in an approximately one percent lower rate of pre-term births (less than 37 weeks gestation). This difference has been consistent over the past 12 years. Over this period of time the pre-term rate has been increasing both in Vermont and nationally. The pattern of increase is the same regardless of which method was used to determine weeks gestation. However the NCHS method results in an approximately one percent higher rate each year. (see graph below).

        This difference was due to NCHS's greater use of the calculated weeks gestation because of their broader birth weight ranges. The NCHS imputation did not affect the pre-term rate. As a result of this finding Vermont has adopted the NCHS algorithm for checking weeks gestation against birth weight so that Vermont reports will be comparable to national data.

        In 2001, 92.5 percent of the records used the calculated weeks, 7.2 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.

WEIGHT GAIN DURING PREGNANCY

        The weight gain guidelines reflected in this report were taken from the Nutrition During Pregnancy, Institute of Medicine, National Academy of Sciences, 1990. The recommendations are as follows:

PRE-PREGNANCY BMIRECOMMENDED WEIGHT GAIN
< 19.828 - 40 lbs
19.8 - 26.025 - 35 lbs
26.1 - 29.015 - 25 lbs
> 29.07 - 15 lbs
Multiple Births35 - 45 lbs


 

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