This was only KN's second child with DN. Normally in non-high-risk pregnancies, one would not have many ultrasounds, so an ultrasound in KN's situation should be somewhat memorable.
We don't know KN's pregnancy status, however she did leave the state only 2 weeks after giving birth, an approx 18 hour drive which may have been more difficult to do if the pregnancy had been high risk... they may have wanted to monitor the child more than usual, etc.
Don't people usually get at least one photo of an ultrasound image to take home with them? Wouldn't that inspire conversation about what the baby looks like at that point, etc?
It isn't like DN had a job he had to attend, so if they were still together as a couple, why *wouldn't* he have gone with her? And if he didn't, did they disagree about him not going?
In the beginning did we not read that DN had just split up with his wife just before the girls were killed? But now KN is intimating that they were together, otherwise she would know for sure that DN had not attended the U/S with her, and she likely wouldn't have driven him the following day for his 'check-in' either. If he was living under a bridge at the time, where was *she* and their young daughter living?
If DN did *not* attend the U/S with her, did he instead babysit their daughter while KN went alone?
Wouldn't all of the above be memorable enough that one could put the pieces together in her memory, based on conversations with each other, conversations with their daughter (perhaps about the ultrasound/her soon-to-come sibling, etc), talk about the photograph, conversation with the U/S technician, banter about the discomfort of a full bladder, etc? There are lots of potential things to remember about that U/S event that it shouldn't be that difficult to remember whether DN was present or not.
Hopefully the U/S technician was interviewed by LE in the hope that something may have been written in her/his notes about the visit, or something which may spark a memory of whether DN was in attendance.
Has it been stated what time the U/S appointment was, how long it took, where it was located, and what happened before and after the appointment? If it was in the morning, then perhaps it is totally insignificant whether or not DN was present?
Just random thoughts..