Back to the question at hand lets see which scenario is most likely.
Rivka felt a conflict within her unborn child. She went to ask a Navi and was told the conflict is among twins.
Rivka felt a conflict within her unborn child. She went to ask a Navi and was told the conflict is among triplets.
Rivka felt a conflict within her unborn child. She went to ask a Navi and was told the conflict is among quadruplets.
Rivka felt a conflict within her unborn child. She went to ask a Navi and was told the conflict is among quintuplets.
Et cetera.