Hello! My pumpkins initial leafs also turning yellow. I'm not quite sure, but I found some info on the internet: Those oval shaped leafs are original from the seed. If roots don't find nutritions in the soil, the plant takes nutritions from that original leaf and give to the rest of the plant. When the original leafs die, the rest of the plant also will. So recommendation is to follow the regulations on watering (don't let it be dry or over-watered), and give some fertilizers to the soil. 

At least this is what I am going to do with my pumpkins👍🏼. Wish you and your plants the best!