Folic acid is the key supplement in early pregnancy because it lowers the risk of neural tube defects such as spina bifida. It is recommended before conception if possible and through at least the first trimester. Some people with specific risk factors are advised a higher amount, which a doctor will determine.
Vitamin D is also commonly recommended throughout pregnancy and while breastfeeding. A general prenatal multivitamin can help cover other nutrients, and iron may be advised if you become anaemic.
Avoid high-dose vitamin A supplements and products like high-dose fish liver oils, as too much vitamin A can harm the baby. Don't assume more is better with any supplement. Discuss what you actually need with your doctor or midwife, especially if you have a medical condition or follow a restricted diet.