Which condition is a contraindication for performing an external version on a fetus?

Engagement of the presenting part is indeed a contraindication for performing an external version on a fetus. When the fetal presenting part — usually the head — is engaged in the mother's pelvis, it indicates that the fetus is in a position optimal for delivery via vaginal birth. Attempting an external version under these circumstances may lead to complications such as cord compression or fetal distress, as there is limited space to effectively manipulate the fetus.

In contrast, conditions such as hydramnios can sometimes facilitate the external version due to the increased amniotic fluid, while a previous cesarean with a low transverse incision and multiple gestations require careful assessment but do not automatically rule out an external version. Therefore, engagement of the presenting part is the most clear-cut contraindication, as it suggests the fetus is positioned in a way that makes an external attempt both difficult and potentially harmful.