Morphstates - Fat, Fit, Thin and Special - are important because most sims you create will feature some combination of fitness values. For example, a sim may have the Fat slider set to 20% and the Fit slider to 10%. The Special morphstate is reserved for maternity clothing.
Teen and elder EA clothing items don't have morphed maternity meshes; while they do have a 'group_special' group that's normally reserved for them, the 'group_special' in teen meshes is, shape-wise, just a copy of the 'group_base' group. This means that if you have a pregnant teen or elder sim, they will either:
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show up with nothing in the body area ("floating head syndrome") if there are no clothes enabled for the Maternity outfit category;
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not show the various stages of pregnancy i.e. growing belly, instead displaying the regular mesh.
If an (young) adult clothing item is not enabled for maternity, the Special morph will, once again, be a copy of the Normal one.
Some ages will not have all morph states usable in-game e.g. children and elders don't use Fit morphs, toddlers don't use any morphs, and the meshes for these unused categories will, once again, be identical to the normal morph state. They can be meshed appropriately, but the game won't use them.
Now that we got that out of the way, let's once again open up the Mesh Toolkit, this time going to the Auto Tools for WSO tab and to the Auto-create Morphs subsection.
As before, in the first field we will load the mesh with the bone assignments, namely LOD1_withBones. In the second field, we will load our reference mesh, the same one we used for the bone assignment (however, you can use a different mesh if it suits your needs better).
Make sure the 'Do interpolations' and 'Interpolate by distance only' are enabled, and click the 'Create Morph Meshgroups' button. I will save the resulting file with the name LOD1_withMorphs.
You can import the mesh into your TSRW project if you want to, but I find it more useful (and less time consuming) to open it directly in Milkshape, in order to check for errors. So, I'm going to create a new Milkshape project and import the 'LOD1_withMorphs' file into it.
If you want your mesh to have a maternity morph, delete the 'group_special' group, export the file again (replacing the LOD1_withMorphs file or naming it something different), and run it through the Mesh Toolkit's 'Auto-create Morphs' option again, using the WSO_tf_Top_Nude_maternity.wso file as reference. Say yes to the "Only missing morphs will be added" message, and save the resulting file over the LOD1_withMorphs one. Now, import this file into a newly created Milkshape project.
After importing, hide each mesh group except for the 'group_fat' one (click on each group and then on the Hide button in the Groups tab).
Once again, we're going to rotate the mesh around and check for any areas where it may be misshapen. Remember that the Toolkit creates morphs based on the bone assignments, but those can't be 100% perfect all the time - ever some EA clothing items have bad fat meshes that clip with themselves or other meshes.
Rotating the mesh, I've already identified some problem areas:
In the case of the last one, aside from those vertices in the middle being more pushed back, there isn't much to do. But we will also take into account the fact that this top is more loose around the front, which means that it shouldn't look like it was suctioned onto the sim's body. Which means that, aside from the skin, I will reshape it to drape more loosely over the sim's body. Again, just because EA meshed the clothing in a certain way, it doesn't mean we have to stick to that; we can reshape it if it would look better that way!
Next, we're going to do exactly what we did in Step 2: move vertices around. I recommend switching between Textured and Wireframe view when selecting vertices, to make sure you've gotten the correct ones.
After we're done with this, we import the new LOD1_withMorphs file into the TSRW project, under the 'High level of detail' section.
We can also use the Fat-Thin and Normal-Fit sliders at the top to take a look at the morphs. Unfortunately, the maternity morph cannot be previewed in TSRW unless you rename the 'group_special' group as another of the groups (e.g. group_fat), but even then the rest of the body will morph in accordance to that type of morph (e.g. a pregnant top but fat head and bottom).
Now that we're done with the morphs, it's time to make the meshes for the other levels of detail (LODs)!