In catholic moral, the marital act during pregnancy is , per se, totally okay, is not sinful, is not wrong.
Objectively and by principle, that is not a reasonnable reason for wife of refusing the request of union of bodies coming from her husband;
Objectively and by principle, that is not a reasonnable reason for wife of refusing to ask the union of bodies to her husband;
Objectively and by principle, that is not a reasonnable reason for husband of refusing the request of union of bodies coming from his wife;
Objectively and by principle, that is not a reasonnable reason for husband of refusing to ask the union of bodies to his wife.
By exception, if there are medical issues (see with a catholic doctor), wife has the moral right to refuse the marital act and the husband has the moral obligation not to ask for it. If wife is tired by her pregnancy, she can say no, of course, because she is tired and not because her situation of pregnancy, per se, is a prohibition coming from the natural law or the catholic law.
It is very necessary to understand that the motives of the conclusion are as important as the conclusion, itself. The rules okay, but with the good intellectual approach, otherwise there would be many bans concerning the union of bodies: absurdity, the " ism " ideologies and the " ism " heresies…etc. The fact of discerning is very important, it is a duty, an obligation