The American College of Obstetricians and Gynecologists recommends you get a booster dose if you are pregnant or if you have recently given birth and you received the Pfizer or Moderna vaccine at least 5 months ago. You can get a booster dose at any time during pregnancy. If you were originally vaccinated before pregnancy and you are now pregnant, you should still get a booster.
When deciding which booster to get, you can choose either of the approved COVID-19 vaccines. You do not need to choose the same vaccine that you originally received.
https://www.acog.org/womens-health/faqs/coronavirus-covid-19-pregnancy-and-breastfeeding (The American College of Obstetricians and Gynecologists)
https://www.massgeneral.org/obgyn/news/covid-19-boosters-pregnant-patients (Massachusetts General Obstetrics & Gynecology)