Reasons for trisomy 13 or 18 births despite the availability of prenatal diagnosis and pregnancy termination.

Background: Few studies have evaluated the reasons why lethal chromosomal anomalies continue to occur despite the importance of this question for maximizing perinatal care.

Objective: To determine why trisomy 13 or 18 births continue to occur in Alaska.

Methods: Case series involving review of maternal and infant medical records. Methods: All 28 known infants and fetuses that died with trisomy 13 or 18 during 1992-2001 and their mothers. Methods: The proportion of mothers that declined or received a variety of routine prenatal tests, the results of prenatal testing, and the impact of testing on decisions related to pregnancy.

Results: Seventeen women declined pregnancy termination or amniocentesis, 10 had no prenatal risk factors and were not offered these procedures, and one woman had an amniocentesis but was not offered pregnancy termination. Twenty-six women had >/=1 prenatal ultrasounds; for 17 women, these were interpreted as normal throughout pregnancy (n=11) or until after 30 weeks gestation (n=6) despite substantial fetal malformations. Fourteen of 15 women with an abnormal ultrasound had an amniocentesis compared to one of eight women whose only risk factor was advanced maternal age.

Conclusions: Most trisomy 13 or 18 deliveries occurred to women who declined amniocentesis or pregnancy termination. Failure to identify abnormalities on prenatal ultrasound may have contributed to the decision not to have these procedures.