Medications in Pregnancy: How Doctors Weigh the Evidence

You just found out you are pregnant, and you take a daily medicine. Or you have a headache, a cold, or heartburn, and you are not sure what is safe. You search online and find answers that conflict. It can feel like every choice is a risk.

This guide explains how clinicians think about medicines in pregnancy, where the evidence comes from, and why stopping a medicine is not always the safer choice.

The evidence gap

Medicine use in pregnancy is very common. The CDC reports that about 9 in 10 women take some medicine during pregnancy, and about 7 in 10 take at least one prescription drug.[1]

Yet the evidence is thin. Fewer than 10% of medicines approved since 1980 have enough information to judge their safety in pregnancy. Pregnant people are usually left out of the studies that test new drugs.[1]

A starting point: background risk

About 1 in 33 babies in the US is born with a birth defect.[2] Most birth defects have no known cause and are not related to medicine. This background risk exists in every pregnancy. A key question for any drug is whether it raises the risk above this baseline, and by how much.

What your doctor looks at

The type of evidence

Since there are few trials, clinicians combine several kinds of evidence:

• Animal studies: useful as a warning sign, but results do not always apply to humans.
• Pregnancy exposure registries: groups that follow people who took a drug during pregnancy and record outcomes.[3]
• Cohort and case-control studies: larger studies that compare outcomes in exposed and unexposed pregnancies.
• Years of experience: older drugs that many people have used in pregnancy often have more reassuring data than new drugs.

Timing

The same drug can carry different risks at different times:

• The first trimester, when organs form, is the main window for structural birth defects.
• Later in pregnancy, some drugs affect growth, the baby's kidneys, amniotic fluid, or the newborn after birth.
• For example, the FDA advises avoiding NSAIDs, such as ibuprofen and naproxen, from 20 weeks of pregnancy unless a clinician advises them. Use at that stage can cause rare kidney problems in the baby and low amniotic fluid.[4]

How labels describe pregnancy risk

US prescription labels used to give each drug a letter: A, B, C, D, or X. Many people read these as grades of safety, but they were often misleading. The FDA's Pregnancy and Lactation Labeling Rule removed the letters.[3]

New US labels have written sections instead:[3]

• Pregnancy: a summary of the human and animal data, the risk of the condition itself, and any pregnancy registry.
• Lactation: what is known about the drug in breast milk and effects on the infant.
• Females and males of reproductive potential: advice on pregnancy testing, birth control, and fertility.

In Canada, Health Canada product monographs also describe pregnancy and breastfeeding data in words, in a section on special populations. In both countries, the wording is a summary, and your clinician still has to apply it to you.

The risk of not treating

Stopping a needed medicine can harm both the pregnant person and the baby. The CDC notes that in some cases, avoiding or stopping a medicine may be more harmful than taking it.[1] Examples of conditions where good control matters:

• Depression and anxiety.
• Epilepsy, where seizures can harm the pregnancy.
• Asthma, where poor control can lower oxygen to the baby.
• High blood pressure and diabetes.
• Thyroid disease.

Some drugs for these conditions are switched before or during pregnancy to options with better safety data. That switch should be planned, not sudden.

Drugs with known strong risks

A small number of drugs have clear evidence of serious harm and need special planning. These include isotretinoin for acne, valproate for seizures and mood, mycophenolate, methotrexate, and warfarin early in pregnancy. Some blood pressure drugs, such as ACE inhibitors, carry risks later in pregnancy. If you take any of these and could become pregnant, talk with your clinician about birth control and planning.

Pain relievers and a 2025 update

Acetaminophen has long been the usual choice for pain and fever in pregnancy. In 2025, US health officials announced plans to add wording to labels about a possible link with autism. Major groups of obstetricians in Canada and the US stated that current evidence does not show that acetaminophen causes autism, and that untreated fever and pain also carry risks. Ask your clinician about the right use for you.

Planning ahead

• Review all your medicines with a clinician before you try to conceive, if you can.
• Take a daily supplement with 0.4 to 0.8 mg of folic acid, starting at least 1 month before pregnancy, as the USPSTF recommends.[6]
• Ask whether your medicine has a pregnancy registry you could join.
• Check breastfeeding safety in advance. LactMed is a free NIH database that summarizes the evidence for medicines in breast milk.[5]

Questions to ask your clinician

• Is it safer to continue, change, or stop this medicine during pregnancy?
• What are the risks of my condition if it is not treated?
• What does the evidence for this drug in pregnancy show, and how strong is it?
• Does the timing in pregnancy change the risk?
• Is there a pregnancy registry for this medicine?
• Which nonprescription medicines are safe for me now?
• Is this medicine safe while breastfeeding?

When to get care quickly

Call 911 or go to the emergency department if you have heavy vaginal bleeding, severe belly pain, a seizure, chest pain, trouble breathing, a severe headache with vision changes, or thoughts of harming yourself. Call your care team right away if you notice your baby moving less than usual.

Bottom line

Medicine decisions in pregnancy weigh two risks: the possible risk of the drug and the risk of an untreated condition. Evidence is limited, so clinicians combine registries, observational studies, and experience. US labels now explain the evidence in words instead of letter grades. Plan any change with your clinician rather than stopping on your own.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.