Sounds like SI joint pain not sciatica. Due to hormones making the ligament at SI joint loose and ready for childbirth and so the joint is unstable. Mine resolved after childbirth. Belly band can help reduce movement at the joint (that’s what my friend did). My SI joint pain got better after asking my husband to massage my glutes (the muscles surrounding the SI joint) every night.
I have them throughout both pregnancies but not as much. Monitor your sitting/ laying down positions and cushioning is important too. Stretching exercises also helps ease the pain.