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ical—i.e., that a pregnant teenager who is still growing may be competing for nutrients with the fetus, or that pregnancy within two years after menarche increases the risk of preterm delivery.4 Psychological factors may also be involved, since many adolescent pregnancies are unplanned, unwanted or discovered late;5 a pregnant teenager may lack the emotional maturity to take responsibility for a pregnancy even after she has decided to carry it to term. And part of the explanation may simply reflect selection bias, since teenagers who become mothers are more likely than others to be poor, to be undereducated or to live in areas with limited access to resources and services.6 Teenagers are not the only age-group at high risk for poor birth outcomes, but they have received the most public attention. Although a constellation of risk factors for adverse birth outcomes applies to teenage mothers, once the effects of these risk factors are controlled for, early childbearing may actually improve infant health in some cases (particularly among blacks).7 On the other end of the age spectrum, while women who give birth relatively late in their reproductive lives have fewer socioeconomic disadvantages than teenagers, they nonetheless share increased risks for poor birth outcomes. Delayed childbearing poses its own biological risks, such as an increased likelihood of medical conditions such as hypertension and diabetes.8 In addition, women aged 35 and Nancy E. Reichman is a research staff member at the Office of Population Research, and Deanna L. Pagnini is assistant professor of sociology and public affairs, both at Princeton University, Princeton, NJ. The authors are grateful to Maryanne Florio and Virginia Dato and to many others at the New Jersey Department of Health for providing the data on which this article was based and for expediting the process in many ways. They would also like to thank the library and support staff at the Office of Population Research. Maternal Age and Birth Outcomes: Data from New Jersey