Thyroid levels? WTH? (oops, I saw that you meant thyroid hormone levels)
Anyway, it makes sence, yet it dosen't. Conception month will mean nothing because the human zygote/fetus is carried for nine months. That's nine months of pure growing, meaning that the introduction of pesticides can occur at any level (date) of pregnancy and affect the growing zygote/fetus, regardless of the conception date.
They are probably right about the introduction of pesticides, though. The growing zygote/fetus needs nothing that the mother cannot provide naturally (ie. normal foods, ect), and the introduction of anything else so rapidly can cause problems.
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