A thin, shortened forearm with an inclination to the ulnar side, dislocation of the radial head, and limited rotational function of the forearm are among the clinical symptoms of congenital ulnar agenesis disease. Autosomal dominant and invisible inheritance is the most fundamental cause, so we must pay attention from the fetal period to prevent the disease. The fetal period is the most important period for the development of the human skeleton. The fetus lives in the environment of the mother’s womb, and the growth and development of the fetal skeleton during the second trimester requires calcium and phosphorus from the mother, and the absorption rate is influenced by the nutritional level of vitamin D from the mother. Vitamin D can influence the differentiation of fetal skeletal cells and promote the deposition of phosphorus in the newly generated fetal bone tissue to form bones. If the mother is vitamin D deficient, it will directly affect the fetal bone development and make the fetal bone calcification impaired, resulting in low fetal weight. Severe maternal vitamin D deficiency can seriously affect fetal bone development and cause fetal bone malformation, resulting in congenital ulnar bone deficiency.