Do Prenatal Vitamins Have Side Effects? What's Normal and What to Do About It

Written by Jess Rosenberg, moode Founder | Reviewed June 2026

When I was formulating The Prenatal, one of the clearest pieces of feedback from women I spoke to was this: they had stopped taking their prenatal vitamin because it made them feel sick. Not because they did not want to take it. Because they could not.

That is a significant problem. A prenatal you cannot consistently take is not doing its job. And in most cases, the nausea, constipation and digestive discomfort that causes women to abandon their supplements has a specific, addressable cause — iron.

Here is what you actually need to know about prenatal vitamin side effects, what is normal, and what is worth addressing.

What are common prenatal vitamin side effects?

Some common side effects of prenatal vitamins are similar to pregnancy side effects, especially nausea and digestive issues. So, if you start taking a prenatal while pregnant, it can be tricky to differentiate between what is causing your discomfort. If you're lucky, you won't get any vitamin side effects at all — it can be different for every person.

Since prenatal vitamins are packed with high doses of nutrients, enough to support you and your baby, if you take a prenatal while not trying to conceive, you may experience side effects as a result of having too much of certain nutrients in your body. That's why it's best to only start taking a prenatal when you're trying to get pregnant, during pregnancy, and during the postpartum phase.

Can prenatal vitamins cause nausea?

Yes, and iron is usually the reason.

High-dose iron is one of the most common irritants in prenatal supplements, and it is also one of the most commonly included ingredients. Iron is added because iron requirements increase significantly during the second and third trimester of pregnancy. The problem is that for many women, particularly in the preconception period and early pregnancy when morning sickness is already a factor, supplemental iron contributes meaningfully to nausea, bloating and stomach upset.

Large capsule size is a secondary factor. A prenatal that is difficult to swallow - either because of its size or its smell - creates its own barrier to consistent use.

What to do: Take your prenatal with food, or directly before bed, to reduce stomach irritation. If nausea is significant and you are also in early pregnancy, taking it at night means you sleep through the worst of the digestive response. If iron is the specific issue, an iron-free formula is worth considering - particularly in the preconception period, when most women are not yet iron-depleted enough to need supplemental iron.

The Prenatal by moode is iron-free by design. We made this decision deliberately - because a prenatal you can actually keep down is more useful than one that is theoretically complete but sits untouched in a drawer. Iron levels are best monitored through blood testing and addressed through food and targeted supplementation when clinically indicated. Always read the label and follow directions for use.

Can prenatal vitamins cause constipation?

Yes - again, typically iron.

High-dose supplemental iron is one of the most well-established causes of constipation. It can also cause the opposite problem - diarrhoea - in women whose gut is already sensitive, or who are experiencing digestive changes from early pregnancy.

What to do: Keep fibre intake high - whole grains, legumes, fruits and vegetables all help. Stay well-hydrated. If constipation is persistent and linked to your prenatal, an iron-free formula is the most direct fix.

Can prenatal vitamins cause green or black poop?

Yes, and it is harmless.

The change in bowel movement colour - green or sometimes black - is a direct result of iron passing through the digestive system. It is simply your body processing the iron, and it is not a sign that anything is wrong. If you are not experiencing any other digestive symptoms alongside the colour change, there is nothing to address.

If the colour change is accompanied by constipation, bloating or significant discomfort, that is the signal to reconsider whether the iron dose in your current formula is appropriate for where you are in your fertility journey.

Why is my urine bright yellow after taking prenatals?

Also normal, and nothing to worry about.

The fluorescent yellow colour comes from riboflavin, also known as Vitamin B2, which contains naturally occurring yellow pigments. Your body absorbs what it needs and excretes the rest through urine. It is simply your kidneys doing their job with water-soluble vitamins.

Can prenatal vitamins cause weight gain?

No. Prenatal vitamins do not directly cause weight gain. Weight gain during pregnancy is a normal physiological process driven by the growing baby, increased blood volume, fluid retention and changes in fat storage - none of which are caused by the supplement itself.

What if I cannot swallow the capsule?

Some prenatal capsules can be opened and the contents added to food. Smoothies, yoghurt and chia pudding all work well. The nutrients are not affected by doing this as they are not heated in the process, and for women with a strong gag reflex or significant nausea, it is a practical solution worth knowing about. moode's prenatal's are designed to be opened if needed.

What should I actually look for in a prenatal?

The supplement market is crowded and it is genuinely difficult to navigate without guidance. The most important variables are:

The form of folate.

Folic acid is the synthetic form and requires conversion before your body can use it. Calcium folinate and methylfolate are active forms that do not require conversion - more accessible for women who carry a gene variant that impairs folic acid conversion.

Choline.

Fewer than 10% of people meet their daily choline requirements, yet it plays a comparable role to folate in supporting neural tube development. Most prenatal vitamins contain little or none. Check the label for a meaningful dose - ideally 300mg or above.

Whether iron is included and at what dose.

If you are in the preconception period or early pregnancy and not iron-deficient, high-dose supplemental iron may be causing unnecessary digestive discomfort. Get your ferritin checked before deciding whether you need it.

Capsule size.

A prenatal you can swallow consistently is more valuable than one that is technically complete but difficult to take.

moode answers your questions about prenatal vitamin side effects

How long do prenatal vitamin side effects last?

Most digestive side effects - nausea, bloating, constipation - settle within one to two weeks as your body adjusts. If they persist beyond that, the formula may not be the right fit for your body. Taking your prenatal with food and before bed are the first adjustments to try.

Is it safe to take a prenatal on an empty stomach?

For most women, no. Taking a prenatal on an empty stomach significantly increases the likelihood of nausea. With food, or immediately before bed, is almost always better tolerated.

Can I take a prenatal if I am not trying to conceive yet?

Yes. A quality prenatal supports your nutritional health broadly, and starting three months before you begin trying is the clinical recommendation - not waiting until you are actively trying. The nutrients in a prenatal, particularly choline, folate and B vitamins, support hormonal health and energy regardless of where you are in your fertility journey.

Why does my prenatal smell strange?

B vitamins have a naturally strong smell, particularly B6 and B12. This is normal and not a sign that the supplement has gone off. If the smell is triggering nausea, taking the capsule with a strongly flavoured food or drink can help.

Should I take my prenatal in the morning or at night?

If you are experiencing nausea, before bed is generally better tolerated - you sleep through the digestive response. If you are not experiencing side effects, the timing matters less than consistency. The same time each day is the most useful habit.

About the author

Jess Rosenberg is the founder of moode and a trained nutritionist and naturopath. She created The Prenatal after her own experience of pregnancy left her questioning the quality of what was available on the Australian market. Learn more about moode.