Short answer, yes. Pregnant women are recommended to receive the MMR vaccine BEFORE pregnancy as this confers protection to the developing fetus from rubella infection. Not having immunity to rubella greatly increases the risk of Congenital Rubella Syndrome (CRS) in the fetus if the mother acquires rubella during pregnancy.

The vaccine should be administered before pregnancy since the MMR vaccine is live attenuated virus, meaning there is a small chance of developing a mild version of the disease. Although the symptoms would be mild for the mother, the risk of CRS for the fetus means it is advised to not receive the vaccine during pregnancy.

In terms of immunity, typically people who have completed the full course of MMR as a child are considered to have life-long immunity. Antibody titers do wane over time, so the only way to confirm would be to measure antibody titers with blood work.