What are the effects of mixed connective tissue disease in pregnancy?

Varies. Connective tissue disorders vary in severity and effect on pregnancy. The most common effect can be decreased blood flow to the baby which can result in problems raning from poor growth, low fluid to stillbirth. It may also increase your risk of high blood pressure of pregnancy. You should talk to your rheumatologist and OB before getting pregnant to optimize your chances of a healthy pregnancy.
It could flare. It could flare but each person is different. It depends on what involvement you have.