Hi ITNEVERRAINS,
Congratulations on your upcoming baby first of all 
The most important first steps of choosing the “best” mattress for you is some initial research into the different materials and combinations that are available so you have a general idea of how mattresses are made, the materials that are used, and what makes better and worse quality in each type of material along with some general information about the types or mattresses that are more suitable for different body types and sleeping styles. Once you’ve done this … then the next step is to to first eliminate the worst choices and find the better retailers or factory direct manufacturers in your area who understand the importance of finding a mattress that matches your needs and preferences and has the knowledge, experience, and willingness to tell you what is in the mattresses they sell. This is the only way to really know the quality and value of a mattress and unfortunately only the minority of retailers will provide this type of information.
Once you have done this … then it’s time to actually start testing mattresses so that you have an idea in “real life” what the different types of mattresses and materials feel like for you. At this point you may know more than some of the people who are selling you a mattress
The steps outlined in post #1 here can help provide some guidance in this and link you to some general information that will help you ask more meaningful questions and can give you a good sense of who is more knowledgeable and interested in actually educating you about the quality and value and suitability of the mattresses they sell and who is more interested in just selling you a mattress without regard to what is in it or how suitable it may be for your unique needs and preferences.
The list of better possibilities in the Nashville area are listed in post #7 here.
With back issues … it’s especially important to test for how well a mattress you are considering keeps you in alignment in all your sleeping positions. This is the main job of the support layers in the mattress. If your heavier parts (like the hips and pelvis) sink down too far, it can aggravate back issues. In addition to this … the softer comfort layers on top need to be soft and thick enough to provide you with good pressure relief and isolate you from the firmness of the support layers. The balance between these two (softness in the upper layers and firmness in the deeper layers) is the reason that different mattresses work well for some but not for others. Side sleepers need a little thicker and softer upper layers because side sleeping has more pressure points than other sleeping positions but the “trick” is to have “just enough” softness and thickness so that support and spinal alignment isn’t compromised. If you are unsure between two mattresses … the firmer choice will generally be better than the softer choice because you fan always “fine tune” a mattress that is too firm with mattress pads or toppers but a mattress that is too soft and doesn’t provide good support/alignment is much more difficult and sometimes impossible to “fix”.
Comfort (pressure relief) is fairly easy to test for because it is what you feel when you first lie on a mattress. Make sure though that you are spending enough time on every mattress that you are seriously considering and are completely relaxed because a mattress can feel very different when your muscles are tense and when they are completely relaxed (like just before you fall alseep). Make sure as well that you test for pressure relief for your pressure points in all your normal sleeping positions. It’s also important to make sure you test mattresses with an appropriate pillow (either yours if it works well for you or a suitable pillow from the store).
Support (spinal alignment) is a little more difficult to test for because poor alignment is generally what you will feel when you wake up in the morning (and either feel no discomfort or pain or your back pains are worse than usual due to sleeping out of alignment). Post #11 here can help you test for this.
Quality and durability is what you will feel a year of more down the road compared to when it was new as the materials soften, compress, or degrade. Because you can’t feel quality or durability at all (and even the cheapest and lowest quality materials can feel great in a showroom) … this is where it’s important to know the details of the layers and materials in your mattress.
The “best” mattress during pregnancy would follow the same “rules” as any other time (the best possible balance between pressure relief and alignment) but as the pregnancy progresses things can change as the body shape and weight distribution changes and because of the general discomfort connected with pregnancy. I would choose a mattress that fit your needs and preferences during “non pregnant times” but add some accessories that can help her accommodate and adjust to the changing needs and conditions of pregnancy. One of the best of these would be either a “body pillow” or a “pregnancy pillow” which can provide some extra comfort and support and offset some of the discomfort involved as the pregnancy progresses.
Hope this helps
Phoenix