First Presidency and Quorum of Twelve make following decisions concerning sex in marriage: "not to unite with a woman in view of imprenation till 7 days after the cessation of the menstrual discharge in order for the most healthy procreation. also that after childbirth if delivered of a son she should continue 40 days in her purification [without sexual intercourse with her husband]. If a daughter she [the new mother] should be 70 days separated as unclean for a man. As to sexual connexion during pregnancy[, do] just as they please about that [-] suit themselves." This is earliest known LDS discussion of what is appropriate in sexual relations of married couples. These rules are based on Book of Leviticus, rather than on current medical writings.
[The Mormon Hierarchy - Extensions of Power by D. Michael Quinn, [New Mormon History database ( http://bit.ly/NMHdatabase )]]