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I understand your concern. Definitely pesticides of any category are harmful. If it can damage the pests, then that will be harmful to the baby. But we humans are bigger in size and damage is not significant, unless sprayed daily for many days, for a long time and directly onto the face. By 18 weeks, the baby is fully formed, so there cannot be any significant damage or long term effects on the baby.
To be on a safer side better to avoid your exposure to pesticides. Do not worry, and your baby will be ok now. I hope I have clarified your query, do write back if any more questions and for your feedback.