Can a dermoid cyst affect a baby's development if not removed? does 7x8x6 dermoid on right ovary neccessarily have to removed during pregnancy?

Possibly: Dermoids can have many different types of tissue in them. If these tissues release hormones, then you could be in for a rough ride should you leave the tumor in place. There are also risks to the surgery, even though loss of the pregnancy is probably small. You should discuss this in detail with you doctor.

Ovarian tumors: As long as the dermoid doesn't twist on its own blood supply (torsion) and cause severe pain, in general they are left alone until postpartum. On occasional ovarian tumors need to be removed in the second trimester, but i always suggest getting a maternal fetal medicine opinion before removing if possible.