Stress or bad water conditions will bring down the immune system and that is why you would see the gill flukes now. This can cause them to stop eating.
A pregnant ray will usually stop eating around 12 hours I found before giving birth give or take some hours. If there are some complications with the unborn pups this can also cause some stress and not eating.
It's a little early for the ray to give birth yet so I would clean the filters, gravel vac the bottom and really up your water changes to 30% everyday and see what happens from there. You can also start the prazi if you have done everything else allready.
Alot of people forget they have to double up on cleaning filters with pregnant rays as they will eat twice as much cuasing alot more waste so your usual filter maintenance routine will just not cut it.
