ELLACRAIG, I also have a cream with USP progesterone from iherb and saw the warning on the label. However I am not sure if they mean the bio-identical progesterone or if they refer to the synthetic progesterone which most of the doctors prescribe. 
The label doesn't say that the cream contains a chemical progesterone. The label says:
"California Proposition 65 Warning: This product contains progesterone, a chemical known to the State of California to cause cancer. Consult with your physician before using this product."
I know that bio-identical progesterone helps prevent breast cancer. So maybe they refer to the synthetic progesterone which (I think) increases the risk of cancer, also like synthetic estrogen.
The label doesn't say that the cream contains a chemical progesterone. The label says:"California Proposition 65 Warning: This product contains progesterone, a chemical known to the State of California to cause cancer. Consult with your physician before using this product."
I know that bio-identical progesterone helps prevent breast cancer. So maybe they refer to the synthetic progesterone which (I think) increases the risk of cancer, also like synthetic estrogen.