Even when relations must be postponed for medical reasons, mikveh immersion is permissible and recommended. Mikveh immersion enables the husband and wife to enjoy other forms of closeness short of relations. Additionally, mikveh immersion facilitates one spouse caring for the other, as when one spouse is ill.
A further consideration is that sometimes the medical assessment of when relations are permissible may change. In such a case, having already immersed could enable the couple to resume relations without delay.
Refuah sheleimah!