Pregnancy is the one area where supplement decisions should be led by your midwife or GP rather than by an article or a shop shelf, because both what to take and what to avoid genuinely matter here. This is a general guide to the shape of it, not personal advice, and anything you take while pregnant or trying to conceive is worth running past your maternity team first.
The two with clear evidence
Folic acid is the one with the strongest case. Taken before conception and through the first twelve weeks, it substantially reduces the risk of neural tube defects like spina bifida. The NHS recommends 400 micrograms a day for most people, and a higher prescribed dose for some (for example if you have diabetes, a higher BMI, or a family history), which is a conversation for your GP. This is the rare case where ordinary folic acid is the recommended form, because that is what the trial evidence used.
Vitamin D is the other. The NHS recommends 10 micrograms a day throughout pregnancy, since deficiency is common and vitamin D matters for both you and your baby's bone development.
Iron, if you need it
Iron demand rises in pregnancy, and many women become low or anaemic, but iron in pregnancy should follow a blood test and your midwife's advice rather than being self-started, both because the dose matters and because routine pregnancy bloods will pick up whether you need it.
What to avoid
This is the part that makes professional guidance important. High-dose vitamin A (retinol) can harm a developing baby, which is why pregnancy multivitamins use a safe form and why liver and high-dose vitamin A supplements are avoided in pregnancy. Most herbal supplements have not been tested for safety in pregnancy, so the default is to avoid them unless your maternity team says otherwise. And high doses of anything are best avoided in favour of a purpose-made pregnancy multivitamin at sensible levels.
The simple version
Folic acid and vitamin D have clear evidence; iron if your bloods show you need it; a pregnancy-specific multivitamin to cover the rest at safe doses; and a careful avoid list led by your midwife. A standard pregnancy multivitamin is built around exactly this. The same St John's Wort caution from the contraceptive guide applies in pregnancy too. To check any supplement against a medication, the free checker will show you the result and the reasoning, but in pregnancy your maternity team is the first call.