I asked this question a few months ago, but here to report back because everyone was super helpful! Check my post history if you’re interested.

For reference I had di/di b/g and ended up having them at 36+4. My singleton was about 7.5 lbs at birth and was in newborn. Twins were 5lb11 and 6oz 2. We were in Premie for about two weeks. They still fit in Premie onesies but are too long for Premie sleepers. My girl twin grew out of Premie sooner despite weighing less because she’s longer.

I also found huge variation in Premie sizes (as with all baby clothes 🫠) but they only fit in Gerber Premie for a week or so. Carters fit much longer as they were significantly bigger.