Is taking progesterone harmful during pregnancy?
No. Not the pure progesterone, or "natural" progesterone. After all, the placenta makes progesterone. A level of 15 to 20 of progesterone is usually the comfort level in early pregnancy. I've never seen any problems of overtreating with progesterone.
Your concerns may be because the progesterone treatment comes with an insert warning that there have been abnormalities in a baby's development with progesterone. Unfortunately, progesterone is part of a group of hormones that the US Food and Drug Administration (FDA) requires to carry this warning. It really applies to estrogen. But you should let your healthcare provider know exactly what you are taking to make sure it's safe.