The chemical is needed to make the thyroid hormones that are required for children's brain development before and after birth.
The recommendations were published in the journal Pediatrics.
People typically get the iodine they need from table salt, in the form of iodide.
The American Thyroid Association and the National Academy of Sciences said pregnant and breastfeeding women should get 290 micrograms of iodide a day.
Women may need to take a supplement with 150 micrograms of iodide to reach that recommended level, but most prenatal and lactation vitamins contain less, according to the Council.
It wrote: "Breastfeeding mothers should take a supplement that includes at least 150 micrograms of iodide and use iodized table salt."
The Council also said women may need to be tested for iodine deficiency if they are vegan or don't eat fish.
Dr. Loralei Thornburg told Reuters Health: "Obviously iodine is critical to the fetal and child brain. Therefore having a diet that's rich in iodine is critical."
But she added: "Although many women are largely iodine deficient, most women do get iodine in the (form) of food.
"This isn't something women should freak out about just yet."