Well, I think it depends on why he went, why you didn’t want him to, what discussion the two of you had had (if any) regarding it. Is he normally a good husband, or is this part of a pattern of thoughtlessness/selfishness?
Do you know why he went? Did you ask him to stay with you and not visit his mum, but he ignored your request? If so, did he give a reason?
Loss of a baby/miscarriage obviously only affects the mother physically, with all the additional pain, complications (short and sometimes long-term) that can come with that, but it affects both parents emotionally. Perhaps spending time with his mother was important to him?
It might have just been thoughtlessness, of course, or perhaps he thought you’d rather spend the day with your mum? He might have thought you would not want a reminder of what you has lost. Or perhaps he was a bit all over the place and made the wrong decision. Who knows his reasons, unless you ask him?
In any case, it is probably not a good idea to stew on this for months on end. You need to either decide to move past it and leave it there, or discuss it with him. Silently seething won’t help you- though could be a good displacement activity instead of grieving your loss- nor will expecting him to know why you are angry. Deal with this, don’t let it fester any longer.