Best mattress to suit pregnant partner and back pain suffering me

I have had a super king size Ikea memory foam mattress for coming on 8 years now. In the last year to two years it has lost the ability to remain firm and dips quite easily after lying in it for a short while. I have a history of back pain which comes and goes, and at the moment I am mostly ok, however each morning my back aches due to the mattress and I am constantly changing positions in my sleep to avoid the mattress dipping too much in one area! I believe a firm mattress would be best for me. A decade or so ago I was sleeping on a futon for a couple of years and that seemed quite helpful. My partner, however, is pregnant (early stages) and I read that a firm mattress is not ideal for pregnant women. Obviously, the pregnancy won't last forever, but at the same time needs to be considered. I weigh 67-70kg. She weighs more or less the same (when not pregnant). My height is 5'11 and hers 5'9. Please let me know your thoughts as to what solutions there are in choosing a new mattress.
Many thanks!