- Evidence of preexisting fetal anomalies that may (1) result in a high probability that the fetus/infant will not survive to the end of the study period (e.g., anencephaly, renal agenesis, or Potter's syndrome); or (2) increase the fetal tissue concentration of zidovudine (AZT) or its metabolites to a toxic level (e.g., neural tube or ventral wall defects).