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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 matches that 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.

        In 2002, 92.4 percent of the records used the calculated weeks, 7.5 percent used the clinical estimate of the weeks, the gestational age was manually imputed for 1 record, and the remaining 9 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. According to this source, the recommended weight gain for very overweight/obese women (pre-pregnancy BMI > 29.0) is "at least 15 pounds". In our 2001 report, we used a range of 7-15 pounds for this category to coincide with the recommendations used by our WIC program. However, beginning in 2002, the recommended weight gain range for very overwieght/obese women was increased to 15-25 pounds to more closely match other reports produced at the Vermont Department of Health. The recommendations are now as follows:


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


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