I didn't encounter it but my gynae mentioned that mucus plug is not a sign of impending labour, the variability of duration to labour is quite large like you pointed out. He asked me to look out for contractions and water bag breaking, as those are sure signs of labour coming soon. As you're pretty late into your pregnancy already, perhaps you can also ask your doctor at the next visit since appointments shd be more regular at this stage too. Hope you have a smooth and safe delivery!
Anonymous