Trends in Birth across High-Parity: MATERIALS AND METHODS
We used the “Natality data files” assembled by the National Center for Health Statistics (NCHS) covering the period 1989-2000. The Natality files contain individual records of all live births that occurred in the United States during the stated period. The procedures for quality control of the data are explained in detail elsewhere. The data source forms the basis for official U.S. birth statistics.
We selected singleton live births >20 weeks for analysis. We defined live-born parity as the total number of live deliveries the mother had experienced. For the purpose of this study, we classified mothers into four parity subgroups: moderate (1-4 previous live births), high (5-9 previous live births), very high (10-14 previous live births) and extremely high parity (>15 previous live deliveries). We merged birth counts into three four-year phases: 1989-1992, 1993-1996 and 1997-2000, as paucity of numbers in some tabulated cells would make meaningful analysis difficult or impossible. cheap viagra professional
The interval between the first day of the last menstrual period (LMP) and the date of birth was used to compute gestational age in completed weeks. Records without the date of the LMP were imputed when there was a valid month and year. Clinical estimate of gestation was used from 1989 to 1998 in the computation of gestational age in those cases where the date of the LMP was not reported or where the LMP date was inconsistent with the birthweight. Approximately 4-5% of the gestational ages during the period were based on this estimate. We restricted our analyses to live births and fetal deaths within 20—44 gestational weeks.
We defined birth rate within a parity subgroup as the total number of births observed in that subgroup divided by the total number of deliveries for that period (including those among nulliparous women) multiplied by 1,000. For example, the birth rate for women of moderate parity comprises all births to these women as the numerator and the total U.S. births for that period as the denominator multiplied by 1,000. We computed birth rates by period of delivery across the entire population and repeated the analysis stratified by age and maternal race. We used the term “rate” in this study to describe crude frequencies across time periods as is universally reported for other pregnancy or birth-associated events (e.g. stillbirth rate, neonatal mortality rate, etc.). cheap levitra professional
In the case of maternal age, three categories were constructed: younger mothers (aged <30 years), mature mothers (30-39 years) and older mothers (>40 years). Because of the rarity of higher gradations of parity among teenagers, our analysis did not investigate teen mothers as a separate entity. Maternal race/ethnicity was considered under three categories: blacks (non-Hispanic), Hispanics and whites (non-Hispanic). Other racial or ethnic groups were not included in the analysis because of paucity of numbers.
We computed trend statistics to assess linear trend by means of the Chi-squared statistics for linear trend. In estimating the association between race/ethnicity and parity status, we used the direct method of standardization to adjust for maternal age. All tests of hypothesis were two-tailed with a type-1 error rate fixed at 5%. This study was approved by the Institutional Review Board at the University of Alabama at Birmingham.
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