Am I obligated to perform 12 rak'ahs of prayer as an act of gratitude to Allah, even though the pregnancy for which I made the vow has not yet occurred?

The ruling depends on your intention at the time the vow was made. If you intended it to be absolute, then you are obliged to fulfill it whenever pregnancy occurs. However, if you intended to restrict the pregnancy to the specific month in which the vow was made, then you are not obliged to fulfill it as long as the pregnancy did not occur in that designated month.