Do pre-existing conditions worsen in pregnancy?

Yes. Preparation for a high-risk pregnancy should begin months in advance of stopping contraception. The best approach is to find out before trying to conceive (ttc) if there are tests to be done or medications to be changed. I advise both partners to make family histories, lists of medications and toxins to which they're exposed, checklists of qs they want to ask and to take them to an mfm specialist.
Yes. Anyone with a serious pre-existing condition should make a pre-pregnancy visit with their OB for counseling. For example, certain blood pressure meds are hazardous in early pregnancy and may need to be changed. Some conditions such as diabetes and anemia can easily get worse or become harder to control. Some auto-immune disease get better (often) during pregnancy but may flare after delivery.
Yes. That is an excellent question. Some conditions such as diabetes and high blood pressure may worsen in pregnancy. Other conditions such as multiple sclerosis may improve temporarily in pregnancy. So you would need to speak to your doctor prior to becoming pregnant to discuss a specific medical condition.
Yes. It really depends on the condition, and is influenced by whether the condition is active or under control. Some often worsen (high blood pressure, diabetes, heart disease) while others may worsen, improve, or remain unchanged (autoimmune disease is one example). Have a consultation with an obstetrician or perinatologist to hear about risks and best-control the condition before getting pregnant.
Yes. I beleive everyone should see an obgyn before pregnancy to go over their personal and family history to determine if they are at increased risks and see if any changes in meds or vaccinations or consultations should be obtained before pregancy.