The situation you described is mostly caused by the incomplete separation of thumb and palm due to the interference of harmful substances during embryonic development. Therefore, the causes of the situation are mostly external factors, generally not genetic, please rest assured.

In addition, during the child's pregnancy, we must pay attention to the safety during pregnancy. Avoid colds, unsafe drugs, and exposure to harmful substances and radiation. Pregnant women should supplement multivitamins and folic acid in moderation, and the intake of nutrition should be rich and balanced. These are of great significance for the delivery of a healthy fetus.

I hope my answer can help you, thank you.