Obstetric delivery may be accomplished by only two methods, vaginally or abdominally. In the management of the pregnancy complicated by a fetal malformation, the choice of delivery method may be made on obstetrical grounds or in belief that one method offers the fetus benefit over the other. That choice must be based on knowledge of the nature of the individual malformation in question, the presence or absence of associated fetal malformations, the presence or absence of fetal karyotype abnormalities, fetal maturity, and fetal presentation. Clear evidence of benefit from cesarean delivery is not available in the case of many malformations that are often considered for abdominal delivery. The infrequency of many of these anomalies typically precludes the accumulation of sufficient experience in any one center or successful completion of randomized trials to evaluate treatment modalities. Most studies therefore are retrospective or flawed by small numbers of patients. Logic dictates that certain malformations that produce sufficient enlargement of fetal structures are at probable risk for dystocia of labor (e.g., severe hydrops, severe hydrocephalus, large sacrococcygeal teratoma) and may benefit from abdominal delivery. Other malformations may predispose the affected fetus to trauma or decompensation during labor and vaginal delivery. It is these anomalies that have the most controversy surrounding the "best" mode of delivery and of which we have attempted to address. Table 4 includes proposed criteria for choosing abdominal versus vaginal delivery in the gestation complicated by congenital anomaly. The birth method in the context of a fetal malformation is a choice optimally made after careful discussions with the patient, pediatrician, and pediatric surgeons. Such discussion should include a careful review of the nature of the anomaly, the optimal prognosis, and the evidence of benefit for a specific birth method. Once the pregnancy in question has been evaluated, appropriate consultations obtained, and the available data reviewed with the patient, the obstetrician again assumes a role of patient advocate. The obstetrician is the counselor, the educator, and the friend the patient needs during such a difficult time. The discussion of birth method in the case of a fetal malformation creates a conflict of interest for the patient, but typically a paradoxic result of this conflict occurs. The patient's welfare, which is usually best served by vaginal delivery, may be in conflict with the fetal concerns, which might benefit from abdominal delivery. The data might be clear but more often the data are less than convincing.(ABSTRACT TRUNCATED AT 400 WORDS)