Is it permissible for me to perform Umrah for myself, then for the fetus in my wife's womb, which is 40 days old?

It is not permissible to deputize someone in worship except for what has been explicitly excluded, such as Hajj and Umrah on behalf of someone incapacitated by death, old age, or an illness from which recovery is not expected. Based on this, performing Umrah on behalf of a fetus is not valid, because it has not performed any of the rituals, and it cannot be analogized to a child who performs what they are able to and is assisted with what they are unable to do.