What is the ruling on a husband having intercourse with his pregnant wife in her vagina from behind, out of fear for her and the fetus?

If the matter is as stated in the question, then it is permissible. Imam Ahmad narrated from Ibn Abbas, may Allah be pleased with him, that Umar ibn al-Khattab said: "O Messenger of Allah, I am ruined." He asked: "What has ruined you?" He replied: "I changed my riding position last night." He did not say anything to him. Then Umar said: "So Allah revealed to him this verse: (Your wives are a place of cultivation for you, so come to your place of cultivation however you wish). So the Prophet, peace and blessings be upon him, said: (Approach from the front, and from the back, but avoid the anus and menstruation)." There is no harm in that, as long as penetration is in the vagina, and this is not restricted by the state of pregnancy.