If you were required to have a prescription in order to get the vitamins, then yes, they are definitely considered a medical expense as a prescription.
If you purchased over-the-counter prenatal vitamins on the advice of your doctor, then the guidance is less clear about whether the cost can be deducted. Below is an excerpt from IRS Publication 502 Medical Expenses (https://www.irs.gov/pub/irs-pdf/p502.pdf)
Nutritional Supplements
You can't include in medical expenses the cost of nutritional supplements, vitamins, herbal supplements, “natural medicines,” etc., unless they are recommended by a medical practitioner as treatment for a specific medical condition diagnosed by a physician. These items are taken to maintain your ordinary good health and aren't for medical care.
