Most seaweed is safe to eat during pregnancy in small amounts, but the type matters enormously. Brown seaweed varieties like kombu, kelp, and wakame contain iodine levels high enough to harm a developing baby’s thyroid, while lower-iodine options like nori and dulse carry far less risk. The key is knowing which varieties to enjoy, which to limit, and which to avoid entirely.
Why the Type of Seaweed Matters
Seaweed falls into three broad groups: brown, red, and green. Brown seaweed contains dramatically more iodine than the others. Tested across dozens of commercial products, brown seaweed averaged 1,651 micrograms of iodine per gram, while red seaweed averaged just 240 micrograms per gram. That’s a sevenfold difference, and it has real consequences during pregnancy.
To put those numbers in context, the recommended daily iodine intake during pregnancy is 220 micrograms, and the tolerable upper limit is 1,100 micrograms per day. A single gram of kombu contains roughly 2,267 micrograms of iodine, more than double the safe upper limit in one bite. Oarweed hits 7,800 micrograms per gram. Nori, by contrast, contains about 18 micrograms per gram, making it one of the safest options available.
Here’s how common seaweed varieties compare in iodine content per gram:
- Nori: 18 micrograms (low risk)
- Dulse: 96 micrograms (low to moderate risk)
- Wakame: 172 micrograms (moderate risk)
- Arame: 400 micrograms (high risk)
- Kombu: 2,267 micrograms (very high risk)
- Sugar kelp: 4,400 micrograms (very high risk)
What Too Much Iodine Does to a Developing Baby
Your baby’s thyroid gland is more vulnerable to iodine overload than yours. In adults, the thyroid has a built-in safety mechanism: when iodine levels spike, the gland temporarily shuts down hormone production, then adjusts back to normal. A fetal thyroid can’t always complete that adjustment. Prolonged excessive iodine exposure during pregnancy can suppress the baby’s thyroid function, leading to fetal hypothyroidism or an enlarged thyroid gland (goiter). There have been documented cases where pregnant and breastfeeding women overconsummed brown seaweed products and their babies became ill.
This is why Australia’s food safety authority advises pregnant and breastfeeding women to eat no more than one serving of brown seaweed per week, and why some guidelines go further, recommending pregnant women avoid brown seaweed altogether.
Nori and Sushi Seaweed Are Generally Fine
Nori, the thin sheets wrapped around sushi rolls, is a red seaweed with very low iodine content. Australian and UK food safety guidance specifically notes that nori and other red and green seaweed varieties are not covered by the brown seaweed warnings. Eaten in normal amounts, nori provides useful nutrients without the iodine risk.
One thing to keep in mind with sushi: the seaweed itself isn’t the concern, but prepared sushi can carry a risk of listeria from the rice or raw fish. If you’re eating sushi during pregnancy, that’s the more relevant food safety consideration.
Skip Hijiki Entirely
Hijiki is a dark, shredded seaweed common in Japanese and Korean side dishes. Unlike other seaweed, the primary concern with hijiki isn’t iodine but inorganic arsenic. Hijiki naturally accumulates high levels of this toxic form of arsenic, which has been linked to liver damage and is classified as a human carcinogen. The Hong Kong Centre for Food Safety advises the public to avoid consumption of hijiki altogether and recommends that restaurants and food manufacturers stop using it as an ingredient. This applies to everyone, not just pregnant women, but the stakes are higher during pregnancy.
Hijiki sometimes appears in prepackaged seaweed salads or as a side dish at Japanese restaurants. Check labels and ask before ordering if you’re unsure.
Kelp Supplements Are Not Recommended
Kelp-based supplements are a particular concern. The iodine content in seaweed supplements varies wildly, ranging from 8 to 560 micrograms per gram across tested products. That inconsistency makes it nearly impossible to control your dose. A review in Nutrition Reviews specifically noted that seaweed supplements are not recommended for pregnant women due to this variability, with kelp-based products flagged as especially problematic.
If you need iodine supplementation during pregnancy, a standard prenatal vitamin with a controlled 150-microgram dose of iodine is a far safer choice. This is the amount recommended by Australia’s National Health and Medical Research Council for pregnant and breastfeeding women.
Nutritional Benefits in Perspective
Seaweed does contain a genuinely impressive range of nutrients. It provides vitamin A, folate, calcium, iron, and omega-3 fatty acids. In theory, those are exactly the nutrients pregnant women need more of. In practice, you’d need to eat over five cups of cooked seaweed daily to meet meaningful intake targets for those nutrients, which is neither realistic nor safe given the iodine concerns. Seaweed works better as a nutritional bonus in your diet than as a primary source of any single nutrient.
Sodium is another factor worth noting. Seaweed is naturally salty. A small 5-gram portion of certain varieties can contain up to 0.26 grams of sodium. That’s not a lot on its own, but seasoned seaweed snacks often add extra salt during processing, and sodium intake is worth monitoring during pregnancy.
Cooking Reduces Iodine Levels
If you cook with brown seaweed occasionally, preparation method matters. Boiling seaweed in water significantly reduces its iodine content. Studies on sugar kelp found that boiling for 20 minutes cut iodine by 75%, and blanching at 80°C for two minutes reduced iodine to just 12% of its original level. Soaking in warm water for one to six hours achieved an 84% to 88% reduction. Simply freezing and thawing, however, did nothing to lower iodine.
This means making a soup with kombu and then removing it is safer than eating dried kombu flakes directly. But even with boiling, high-iodine varieties like kombu and sugar kelp can still deliver more iodine than is ideal during pregnancy, so these methods reduce risk rather than eliminate it.
Practical Guidelines
The simplest approach during pregnancy is to stick with nori and other low-iodine varieties in moderate portions. If you enjoy seaweed regularly as part of your cultural diet or eat a plant-based diet where seaweed is a key iodine source, it’s worth paying attention to which species you’re consuming. Vegan diets in particular have been associated with excessive iodine intake from seaweed, because seaweed often serves as the primary iodine source and intake is harder to calibrate than with iodized salt or dairy.
Avoid hijiki regardless of preparation. Limit brown seaweed like kombu, wakame, and kelp to no more than one small serving per week, and boil it thoroughly before eating. Skip kelp supplements entirely. Nori sheets, seaweed salads made from red or green varieties, and small portions of roasted seaweed snacks are all reasonable choices when eaten in moderation.