My wife has intramural fibroid of 21 mm is it a serious condition or nothing to worry. we are trying for pregnancy can this be a factor in infertility?

Very small: Fibroids are benign muscular tumors of the uterus. Yours is so small it is unlikely to cause problems. Of course you will need to discuss with your doctor who did your sonogram and examination.

Don't worry: Probably too small to worry about at this time, especially if not causing symptoms. Possible problem in future depending on location and size.