Relations are fully permitted during pregnancy. The husband's obligation to his wife to have relations at regular intervals (e.g., a minimum of once a week on leil Shabbat), onah, is not waived during pregnancy without the wife's consent. In fact, the Talmud even mentions that relations from the second trimester on have a positive effect on the baby. (Note that this statement in itself does not constitute an obligation.)
Perhaps you should have an open discussion with your husband, to clarify and address his concerns about relations during pregnancy. Other positions for vaginal intercourse are permitted and might ease discomfort. It is also worth noting that you are likely to be in niddah for approximately two months after childbirth. You may wish to contact your physician so that you can reassure your husband on a medical level as well.