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.

I think it depends - the pregnant women may or may not have persisting immunity against rubella.

If she does have immunity, then there is supposed to be protection from rubella, as there is a fraction of her rubella antibodies that is transferred to the fetal circulation through the placenta.

In Canada (or at least the province I'm in), it's routine to check rubella antibody levels in the a women's blood prior to or during early pregnancy, and see if they correspond to levels indicating immunity. The mother may have immunity from an actual previous exposure/infection with the virus, or from vaccination. There are certainly cases where the result has come back as no or indeterminate immunity, rather than suggestive of immunity.

Since the MMR (measles, mumps, rubella) vaccine is a live-virus vaccine, and the big reason to care about rubella in pregnancy is birth defects, the MMR vaccine is not recommended within a few weeks of starting to try and get pregnant, or during pregnancy.

All that said, I think the general recommendation is for pregnant women (immunized or not) is to avoid people with infectious diseases when possible.

Short answer, yes.

Long answer: while in the womb, the mothers immune system protects the fetus. However, maternal rubella antibodies are also transmitted via the placenta to the fetus, protecting the baby after birth. The transfer increases towards the end of pregnancy, so preterm babies have lower levels, making them more vulnerable.

https://pubmed.ncbi.nlm.nih.gov/10378133/

There are other antibodies that are transferred to the fetus: measles, covid, diphtheria*, pertussis*, polio, chickenpox, and others.

*diphtheria, pertussis and tetanus requires the vaccination of the mother during pregnancy.

There is a term in German, Nestschutz - “nest protection” that refers to the passive protection babies have in their early months against some diseases. It wanes as the maternal antibodies degrade, requiring vaccines for the baby.

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