I've been subbing this exercise my coaches showed me for sit-ups at CF, but nobody really knows what it's called.
Basically, I am on all fours on the floor, and will raise my left arm/right leg up above parallel, put them down, then do right arm/left leg. It works my hamstrings and shoulders.
Bird dogs. At least, that's what my gym calls them.
I tried to ETA this, but I've been too lazy to create an account, and you can't edit as a guest.
Just saw your ticker and wanted to mention that I did bird dogs as my only core exercise during 3rd tri, and I think it made a huge difference in my recovery. I had a c-section 11 weeks ago, and my ab strength came right back. I've had virtually no pain since 2 weeks pp, and I was able to start core work right away. Keep them up!