Is thiamine safe during pregnancy?

What does the evidence say about thiamine during pregnancy?

1. Pregnancy raises thiamine needs to about 1.5 mg a day, well above the roughly 0.4 mg per 1,000 kcal baseline used outside of pregnancy. 1
2. Nordic nutrition guidelines recommend an additional 0.4 mg of thiamine a day during pregnancy on top of the non-pregnant baseline, and studies measuring thiamine directly in the blood confirm pregnant and breastfeeding people have a measurably higher requirement than other adults. 2
3. The body stores only about an 18-day supply of thiamine, so severe, prolonged vomiting can deplete it within a few weeks, and hyperemesis gravidarum itself further lowers thiamine levels, compounding the shortfall. 3
4. Left unrecognized, thiamine deficiency from hyperemesis gravidarum can progress to Wernicke's encephalopathy in an estimated 0.1% to 0.5% of severe hyperemesis cases. Early thiamine repletion led to full recovery in more than 80% of cases, while delayed repletion was linked to lasting cognitive damage or fetal loss. 1
5. In one study population, diets heavy in polished white rice, especially when the rice was rinsed several times before cooking, a practice that washes away thiamine, were linked to lower measured thiamine levels, showing that everyday diet affects thiamine status alongside pregnancy's added demand. 4

Is thiamine safe in each trimester?

• First trimester. Morning sickness often starts here, and mild nausea alone isn't a thiamine concern since your prenatal vitamin covers the increased need. If vomiting becomes severe and persistent, thiamine stores can start running low after about three weeks of poor intake, since the body only holds roughly an 18-day supply.
• Second trimester. Thiamine-deficiency complications from hyperemesis have most often been reported around 14 to 16 weeks, after several weeks of ongoing severe vomiting. This is the window where a provider may check thiamine levels or recommend supplementation if vomiting hasn't let up.
• Third trimester. By the third trimester, hyperemesis has usually resolved and thiamine needs are simply met by a balanced diet plus a standard prenatal vitamin. If nausea and vomiting are still significant this late, it's worth flagging to your provider rather than assuming it will pass on its own.