Final answer:
For a breech presentation, the criteria for considering a trial by vaginal delivery is a frank or complete breech with estimated fetal weight less than 3500 grams. These conditions are assessed to minimize the risks associated with breech vaginal deliveries.
Step-by-step explanation:
The criteria for trial by vaginal delivery in cases of breech presentation primarily considers the type of breech and the estimated fetal weight. The most favorable conditions for attempting a vaginal breech delivery include either a frank breech or a complete breech presentation. A frank breech has the legs pointing upward and a complete breech has both legs crossed and pointing downward. Furthermore, the estimated fetal weight should be less than 3500 grams to consider a vaginal delivery attempt. This is because a larger fetal weight may increase the risk of delivery complications.
In the context of the options provided, a) Frank or complete breech, estimated fetal weight < 3500 grams is the correct criterion for considering a trial of vaginal delivery in breech presentation. The fetal weight is significant because it correlates with the likelihood of delivery complications—the heavier the fetus, the greater the risk involved with a vaginal breech delivery.