A brief, low-grade fever during pregnancy is common and usually not harmful on its own, but a sustained fever above 101.3°F (38.9°C) lasting 24 hours or more has been linked to an increased risk of miscarriage, birth defects, and stillbirth depending on the trimester. The key is how high the fever gets, how long it lasts, and how quickly you bring it down.
Why Fever Matters More During Pregnancy
Your body temperature directly affects your developing baby, especially in the first trimester when the brain and spinal cord are forming. Elevated heat interferes with how cells build proteins, which can lead to cell death, disrupted blood flow to the placenta, and problems with how fetal tissue develops. This is why prolonged high fevers carry more risk than a temperature that spikes briefly and comes back down.
The threshold that raises concern is a sustained temperature above 101.3°F (38.9°C) for at least 24 hours. Below that, or for shorter durations, the risks drop considerably. A reading of 100.4°F (38°C) is technically a fever, and the CDC lists it as a maternal warning sign worth acting on, but the association with birth defects and pregnancy complications becomes much stronger at higher, longer-lasting temperatures.
First Trimester: The Highest-Risk Window
Fever during the weeks around conception and the first trimester carries the most studied risk. Research published in the Annals of Epidemiology found that fever in this period roughly doubled the odds of neural tube defects like spina bifida (an odds ratio of 2.4). Neural tube defects occur when the tissue that forms the brain and spinal cord doesn’t close properly, and this process happens very early, often before many people even know they’re pregnant.
Folic acid appears to offer meaningful protection. In that same study, women who took at least 400 micrograms of folic acid daily had a much smaller increase in risk from fever (odds ratio of 1.8, which was not statistically significant) compared to women with low folic acid intake, whose risk jumped more than fourfold. If you’re pregnant or trying to conceive, this is one more reason a prenatal vitamin with folic acid matters.
Later in Pregnancy
Once past the first trimester, the risks shift. Fever in the second and third trimesters is less likely to cause structural birth defects, but it can trigger preterm labor. A sustained high fever later in pregnancy has also been associated with stillbirth. The concern at this stage is less about how the baby is forming and more about whether the fever signals an infection that could affect labor or the placenta.
Common Causes of Fever in Pregnancy
Most fevers during pregnancy come from the same sources as fevers outside of pregnancy: colds, the flu, urinary tract infections, and stomach bugs. UTIs are especially common in pregnancy because of changes in how the urinary tract functions, and they can cause fever if they progress to a kidney infection.
During labor itself, fever most often has one of two causes: an infection of the amniotic fluid and membranes (called chorioamnionitis) or a side effect of an epidural. True maternal sepsis is rare, occurring in only about 1.4% of women who develop chorioamnionitis at term. Still, any fever during labor is taken seriously because it affects decisions about antibiotics and newborn monitoring.
How to Bring a Fever Down Safely
Acetaminophen (Tylenol) remains the safest over-the-counter option for reducing fever during pregnancy. Both aspirin and ibuprofen (Advil, Motrin) have well-documented adverse effects on the fetus and should be avoided, particularly after 20 weeks. The FDA has recommended that clinicians weigh the benefits of acetaminophen against using it routinely for every minor temperature increase, but when a fever is clearly elevated, bringing it down is the priority.
Non-medication approaches can help as well, especially while waiting for acetaminophen to take effect:
- Stay hydrated. Drink plenty of water or low-sugar fluids. Avoid caffeine, which can be dehydrating.
- Cool your skin. Mist or sponge your skin with water, especially while using a fan. Skip ice baths, which can cause shivering and actually raise your core temperature.
- Wear loose clothing. Lightweight, breathable fabrics and cotton bedsheets help your body release heat.
- Eat water-rich foods. Watermelon, cucumber, berries, and spinach all contribute to hydration.
- Rest in a cool environment. Keep blinds closed during the day, improve airflow by opening interior doors, and avoid using heat-generating appliances like the oven.
Warning Signs That Need Immediate Attention
A fever on its own warrants a call to your provider, but certain combinations of symptoms mean you should seek care right away. The CDC identifies these as urgent maternal warning signs when they occur alongside or independent of fever:
- Decreased fetal movement. A noticeable drop in how much or how often the baby moves.
- Severe belly pain. Sharp, stabbing, or worsening cramping, especially if sudden.
- Vaginal bleeding or fluid leaking. Anything beyond light spotting, or a gush or trickle of fluid.
- Severe headache. One that won’t go away, worsens over time, or comes on suddenly with intense pain.
- Vision changes. Flashes of light, blurry or double vision, or blind spots.
- Trouble breathing. Sudden shortness of breath, chest tightness, or difficulty breathing while lying down.
- Severe nausea or vomiting. Inability to keep fluids down, especially with confusion or headache.
Any fever of 100.4°F (38°C) or higher is worth reporting to your doctor or midwife, even if you feel otherwise fine. If your temperature reaches 101.3°F (38.9°C) and isn’t responding to acetaminophen within a few hours, or if it’s been elevated for most of a day, that’s a situation that calls for prompt medical evaluation. The goal isn’t to panic over every warm forehead. It’s to act quickly enough that a treatable infection or a manageable fever doesn’t become a prolonged one.