No, you are not required to drop your Marketplace plan. You have a choice between your Marketplace plan and the Medicaid coverage. You can continue on your plan throughout your pregnancy and continue receiving the premium tax credit. You can also remain on your Marketplace plan and receive premium tax credits if you live in a state where your unborn child qualifies for CHIP coverage.
If you opt for pregnancy-related coverage under CHIP that is tied to your unborn child and choose to drop your Marketplace plan, you will qualify for a Special Enrollment Period (SEP) to re-enroll in Marketplace coverage once you no longer qualify for public coverage through Medicaid or CHIP. (45 C.F.R. 155.420(d); IRS, Notice 2014-71.)