Ah, I think I might have figured it out. Yuki knew she was going to die from childbirth complications. As Zreth pointed out, if you look at the baby picture again it looks more like the baby is coming out of a birth canal, even in the breach position, instead of wearing a hat.
But then the baby's outfit was bothering me. Looking at it a few times, I was hit with the thought that it was supposed to be blood. Yuki's blood. But that shouldn't be that bad for a vaginal birth.
So then I looked at the old woman a few times. I suddenly recalled that Ren said their midwife, the one who kept reassuring them, was a veteran midwife. This picture, then, might be that midwife distraught that the situation is so much worse than she realized.
That's when I realized it.
It was a C-section. The reason for the pillow that Zreth said represented the belly and the baby's position compared to it is that they had to cut it out. Whether only from the breach position, or perhaps Yuki's age. We know she is six years older, but we don't know how old Ren is, so there is a chance she is past the age where all pregnancies are considered high-risk. And that age is, unfortunately, lower than one would think, starting in the mid to late 30s, according to internet research.
This even explains the picture of the woman who is obviously Yuki. Note the pink coloring. White is the normal clothing color for mourning and, I believe, for the body to be dressed in. And that dress is stained pink with blood.
What Ren realized was that Yuki knew she was going to die, and never once told him. She let him be hopeful of a future as a family of three, and never told him. She left him hints and didn't correct his mistaken assumptions, and let him think it just happened when she had made peace with it before it happened. Perhaps she even requested the C-section despite the risk because the baby, to her, deserved to live.
And the reason for that is simple. To not blame the baby. That's the reason for the fourth and fifth pictures. To show him the future where, even without her, he and their child are a happy family. That things would be okay. This may even be why Ren finally realized the truth. Perhaps the two of them were mimicking the scene without knowing and he looked at the picture later, or the child asked how mommy knew what they would be wearing.
Ren can't forgive herself for not telling him, for deciding to put the baby before herself without ever letting him know it was a choice. But he will continue to love her, because Yuki also made sure he only saw the baby as their child and not her killer.
That's my theory, at least.