Folic acid and vitamin D dose checker
Works out whether you need the standard 400 microgram folic acid dose or the prescription-only 5 milligram dose, using the exact criteria the NHS lists. It also decides whether vitamin D is a winter supplement or an all-year one for you, converts between micrograms and international units, and names the supplement to avoid entirely.
Folic acid and vitamin D dose checker
No sign-up · PrivateVitamin D advice is seasonal: 10 micrograms a day between early October and late March at minimum.
Each of these changes vitamin D from a winter supplement into an all-year one.
What to take
The standard NHS dose. Take it from when you are trying to conceive — ideally 3 months before — or as soon as you find out you are pregnant, and through the first 12 weeks. The NHS notes that taking it for longer than 12 weeks will not harm your baby.
Which folic acid criteria you met: None of the higher-dose criteria. The standard 400 microgram dose applies.
Vitamin D: 10 micrograms a day. We are inside the October-to-March window when the NHS says everyone who is pregnant should take it, because UK sunlight is too weak to make enough.
Units, because the labels are inconsistent: Folic acid: 400 micrograms is 0.4 mg. Some packs say mcg, some say μg, some say 0.4 mg — all the same thing. Vitamin D: 10 micrograms is 400 IU. If a bottle only gives IU, 400 IU is the figure you want. The NHS upper limit is 100 micrograms (4,000 IU) a day; more than that could be harmful.
Iron: No universal dose, deliberately. The NHS checks iron levels in routine blood tests at the booking appointment and at around 28 weeks, and a GP or midwife recommends supplements if they are needed. Iron supplements taken without a reason cause constipation and are not harmless — wait for the blood test.
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How this is calculated
FormulaFolic acid: the standard dose
NHS guidance on vitamins, supplements and nutrition in pregnancy: take 400 micrograms of folic acid every day, from when you are trying to conceive — ideally 3 months before — or as soon as you find out you are pregnant, through the first 12 weeks. The NHS adds: "If you take it for longer than this, it will not harm your baby."
Folic acid: the higher dose, and exactly who needs it
A higher dose of 5 milligrams a day, prescribed by a GP, is recommended if you are planning a pregnancy or are less than 12 weeks pregnant and any of the following applies: you or the baby's father has a neural tube defect; you or the baby's father has a relative with a neural tube defect; you have had a previous pregnancy affected by a neural tube defect; you have diabetes; you have a blood condition such as sickle cell anaemia or thalassaemia; or you take medicine for epilepsy or HIV.
Five milligrams is 12.5 times 400 micrograms. It is not available over the counter and cannot be reached by taking more of the shop-bought tablets.
Vitamin D
Take 10 micrograms of vitamin D every day between early October and late March, because UK sunlight is too weak in those months to make enough. Take it all year if you usually cover most of your skin when outside, or if you spend a lot of time indoors; and consider taking it all year if you have black or brown skin, as this affects how much vitamin D your skin makes.
Ten micrograms equals 400 international units. The NHS upper limit is explicit: do not take more than 100 micrograms of vitamin D a day, as it could be harmful.
Vitamin A
Do not take cod liver oil, or any supplement containing vitamin A, sometimes labelled retinol, during pregnancy. Too much vitamin A can harm your baby's development. Foods very high in vitamin A, such as liver, should also be avoided.
Iron
No universal dose is given, deliberately. Iron levels are checked in routine blood tests at the first midwife appointment and at around 28 weeks, and a GP or midwife recommends supplements if they are needed. This tool therefore does not put a number on iron.
How the seasonal rule is applied
The month you select decides the vitamin D answer. October to March inclusive counts as the window in which the NHS advises everyone who is pregnant to supplement. Selecting any of the all-year factors overrides the season and returns an all-year answer regardless of month.
The supplement decision that gets made wrong
Almost everyone knows about folic acid. Far fewer people know that there are two doses, that the bigger one is twelve and a half times the smaller one, and that it is prescription-only. Someone taking epilepsy medication who buys the standard tablets in a supermarket has done the responsible thing and still has the wrong dose.
The list of who needs the higher dose is short and specific, and this tool asks it question by question.
Why the timing is the hard part
The neural tube — which becomes the brain and spinal cord — closes by about 28 days after conception. For most people that is around the time a period is first missed. By the time a test shows two lines, the window folic acid is protecting has largely closed.
That is the entire reason the advice is to start when you begin trying rather than when you succeed, ideally three months before. It is also why the advice sits awkwardly with real life: roughly half of pregnancies are not planned to that timetable, and nobody who missed the window did anything wrong.
If you are already pregnant and have not been taking it, start today and mention it at your booking appointment. That is the correct response, and it is a very common conversation.
The higher dose list, in plain terms
You need 5 milligrams if you or your partner has a neural tube defect such as spina bifida, or has a relative with one; if a previous pregnancy was affected by one; if you have diabetes; if you have a blood condition such as sickle cell anaemia or thalassaemia; or if you take medicine for epilepsy or HIV.
Two of those are worth pausing on. Diabetes means diabetes you already have, not gestational diabetes diagnosed later in this pregnancy — the folic acid window has usually closed by then. And the epilepsy medication entry is there because some anti-epileptic drugs interfere with folate metabolism, which is a genuinely different mechanism from the others on the list.
If any of these apply, the next step is a GP appointment. It is not a supermarket.
Micrograms, milligrams, mcg and that Greek letter
Supplement labelling is a mess and it causes real confusion. Four hundred micrograms is the same as 0.4 milligrams. Some packs write it as 400mcg, some as 400 followed by a Greek mu and a g, some as 0.4mg. They are all the same dose.
Vitamin D has the same problem in a different currency. Ten micrograms is 400 international units. If a bottle only gives IU, 400 IU is the figure you are looking for. Bottles offering 1,000 or 2,000 IU are not what the NHS advises in pregnancy, and the ceiling is firm: do not take more than 100 micrograms — 4,000 IU — a day.
Vitamin D is a season, not a supplement
Between early October and late March, UK sunlight is too weak for anyone's skin to make enough vitamin D, whatever the weather does. That is the window in which the NHS advises 10 micrograms a day in pregnancy.
Three situations turn it into an all-year supplement instead: if you usually cover most of your skin outdoors, if you spend a lot of time indoors, or — the NHS says consider it — if you have black or brown skin, because more melanin means less vitamin D produced from the same sunlight. Those are not edge cases; between them they cover a very large share of the population.
What a pregnancy multivitamin is and is not
A dedicated pregnancy multivitamin is a convenience, not a requirement. The two things the NHS actually recommends are folic acid and vitamin D, and both are available as single supplements for a fraction of the price of a branded pregnancy pack.
If you prefer a combined product, two checks are worth doing. First, that it contains at least 400 micrograms of folic acid and 10 micrograms of vitamin D, because some do not. Second, that it contains no vitamin A or retinol. A multivitamin bought for general use before pregnancy very often does, and swapping it out is one of the first things to do.
The one to actively avoid
Cod liver oil, and anything containing vitamin A or retinol. It reads like the healthiest thing on the shelf and it is the one supplement the NHS tells pregnant women not to take, because too much vitamin A can harm a baby's development. Check the label of any multivitamin you already own: "pregnancy" on the front is not a guarantee, and general-purpose multivitamins frequently contain retinol.
The same logic applies to food. Liver and liver products are extremely high in vitamin A and are on the avoid list for the same reason.
Iron, and why there is no number here
People expect an iron dose and this tool deliberately does not give one. Your iron is checked at the booking appointment and again at around 28 weeks, and a supplement is recommended if the blood test says so. Taking iron without that indication causes constipation, which is already one of the least popular parts of pregnancy, and it is not harmless. Wait for the result.
Getting them free
In the UK you may be able to get free vitamin supplements through the Healthy Start scheme if you qualify, and the vitamins it provides are exactly these: folic acid and vitamin D at the doses on this page. It is worth five minutes checking eligibility, particularly if you are buying both monthly.
This tool applies published NHS advice to your answers. It does not know about any other medicines you take, and it does not replace your midwife, GP or pharmacist.
Sources
- Vitamins, supplements and nutrition in pregnancy — NHS, accessed
- Vitamin D — NHS, accessed
- B vitamins and folic acid — NHS, accessed
- Maternal and child nutrition (PH11) — NICE, accessed
- Planning your pregnancy — NHS, accessed