摘要：Objectives. We evaluated the presence of misclassification bias in the estimated effect of parental involvement laws on minors’ reproductive outcomes when subjection to such laws was measured by age at the time of pregnancy resolution. Methods. Using data from abortion and birth certificates, we evaluated the effect of Texas's parental notification law on the abortion, birth, and pregnancy rates of adolescents aged 17 years compared with those aged 18 years on the basis of age at the time of pregnancy resolution and age at conception. Results. On the basis of age at the time of the abortion or birth, the law was associated with a fall of 26%, 7%, and 11% in the abortion, birth, and pregnancy rates, respectively, of 17- relative to 18-year-olds. Based on age at the time of conception, the abortion rate fell 15%, the birth rate rose 2%, and the pregnancy rate remained unchanged. Conclusions. Previous studies of parental involvement laws should be interpreted with caution because their methodological limitations have resulted in an overestimation of the fall in abortions and underestimation of the rise in births, possibly leading to the erroneous conclusion that pregnancies decline in response to such laws. Over the past 2 decades, 22 states have passed laws restricting minors’ access to abortion, and 36 states now enforce laws requiring parental involvement in minors’ decisions to obtain abortions. 1 The popularity of such laws stems in part from the belief that these laws reduce abortion and pregnancy rates among adolescents. This belief is supported by previous studies that have reported that after the passage of a parental involvement law, there was a fall in abortion rates among minors that was unaccompanied by a rise in births. 2 – 6 However, most previous analyses have used the pregnant adolescent's age at the time of the birth or abortion to classify adolescents as either affected or unaffected by the parental involvement law, and this may have biased estimates of the effect of such laws on the reduction in minors’ abortion rates and the lack of increase in their birth rates. These biases have significant implications for policy. If after a parental involvement law, abortion rates fall less than legislators expect and birth rates rise, then the increase in unintended childbearing by minors may dampen enthusiasm for such statutes. A recent analysis of Texas's parental notification law found that many 17-year-olds who are contemplating having an abortion will, if feasible, wait to do so until their 18th birthday to avoid having to notify their parents. 7 Some of them are willing to wait even if it leads to a riskier second-trimester abortion. Such behavior introduces a misclassification bias into the estimated impact of the law on minors’ abortion rates. The canonical research design for the evaluation of parental involvement laws is a pre–post analysis with a comparison group, in which changes in abortion or birth rates among minors are compared with changes among older adolescents or changes among minors in a similar state without a parental involvement law. The delay in the timing of abortion by older 17-year-olds causes a decline in the number of abortions obtained at age 17 years and a rise in the number of abortions obtained at age 18 years, which leads researchers to overestimate the effect of the law on the abortion rate of 17-year-olds, if exposure to the law is determined based on the adolescent's age at the time of abortion rather than age at the time of conception. Misclassification bias will be more notable if the comparison group is older adolescents in the same state, because the delay of abortions by 17-year-olds will spuriously reduce the number of abortions performed on 17-year-olds and at the same time increase the number of abortions performed on 18-year-olds. If minors in states without a parental involvement law serve as the counterfactual, the bias results from excluding the delayed abortions from the analysis altogether. The way exposure to the law is determined also affects inferences about the effect of the law on minors’ birth rates. By exposure we mean adolescents are subject to the law on the basis of their age. After the introduction of a parental involvement law, adolescents younger than 18 years are subject (exposed) to the law, regardless of whether or not they are aware of the law before they become pregnant. Adolescents 18 years and older are not subject to the law (are not exposed), because the parental involvement requirement does not apply to them. Approximately three fourths of all minors who conceive as 17-year-olds give birth when they are 18 years of age. Thus, a pregnant 17-year-old who carries to term because of a parental involvement law will most likely give birth when she is aged 18 years. In all but 1 previous analysis of parental involvement laws, the mother's age has been measured at the time of delivery. Thus, births to 18-year-olds who may have been affected by the law during pregnancy will not be counted if comparisons are between 17-year-olds in different states, or such births will be included among the controls if changes among 18-year-olds within the state serve as the counterfactual. This form of misclassification bias drives estimates of the law's impact on birth rates toward the null and may even lead to the erroneous inference that birth rates have declined or remained unchanged in response to the law. We sought to evaluate the extent of misclassification bias in estimates of the effect of parental involvement laws if exposure to the law was determined on the basis of the pregnant adolescent's age at the time of abortion or birth rather than her age at the time of conception. We were able to accomplish this objective because of unique data from abortion and birth certificates from Texas, which enacted a parental notification law in January 2000. The Texas data provide precise information that allowed us to determine the adolescent's age at the time of conception as well as her age at the time of pregnancy resolution (abortion or birth). Initially, we followed previous work and used age at the time of abortion or birth to classify adolescents as affected and unaffected by the law. We evaluated the effect of the Texas parental notification law on the abortion, birth, and pregnancy rates of 17-year-olds using 18-year-olds as the comparison group. We then repeated the analysis but used age at conception to define exposure and contrasted the 2 sets of estimates.
