Could I sue an emergency room doctor for a miscarriage?

No, they did not cause the miscarriage.

So the doctor made a mistake. How did that harm you? To prevail in a legal action against the doctor, you must show that s/he owed you a duty, that s/he violated that duty by treating you below the standard of care, and that the erroneous treatment was the direct or proximate cause of identifiable harm. Your question does not show any harm that the law can compensate. Perhaps there are more facts than you imparted in your question. If so, try asking it again with more information. But if there is nothing more, why contribute to the litigation explosion with a frivolous lawsuit?

For what? You already had miscarried before you went to the ER so they did nothing as to your physical health. You feel insulted by the Dr., but he voiced his honest opinion, even though it was wrong. You have no damages.

You very well may have a lawsuit. However, it takes the opinion of a medical expert to determine whether malpractice occurred. Lawyers that do medical malpractice cases typically have such experts available. Consider consulting one. Good luck.

What damage did the doctor cause? Whether or not the ER doctor diagnosed your condition, the miscarriage was spontaneous and he could not have prevented it. It is not unusual for a spontaneous miscarriage to occur within the first trimester of pregnancy, without any trauma.

Doubtful, what are your damages and how were they caused by the ER?