Can eating cornstarch cause preterm labor?

No. Think of corn starch as ground-up corn, potato starch as ground-up potatoes, and rice flour as ground-up rice. Eating any of such starches as part of one's healthy diet does not change one's chances of going into preterm labor. Likewise, omitting one or more types of flour or starch does not increase or decrease the likelihood of a preterm delivery. People should eat a balanced, healthy diet.