Early pregnancy does not cause a true fever, but it does raise your baseline body temperature slightly. After ovulation, your body temperature increases by less than half a degree Fahrenheit (about 0.3°C), and if you become pregnant, that small rise persists instead of dropping back down before your period. This warmth is normal and hormonal. A reading of 100.4°F (38°C) or higher is a clinical fever and is not a typical effect of pregnancy itself.
Why You Feel Warmer in Early Pregnancy
The hormone progesterone is responsible. After ovulation, your ovaries ramp up progesterone production. If an embryo implants, progesterone keeps climbing rather than tapering off. Progesterone acts on the temperature-regulation center in the brain, nudging your internal thermostat upward. This is why many women in the first trimester report feeling flushed, overheated, or slightly warm to the touch even when no illness is present.
If you’ve been tracking your basal body temperature (the reading you take first thing in the morning before getting out of bed), a sustained rise lasting 18 or more days after ovulation can actually be an early indicator of pregnancy, according to the Mayo Clinic. That rise is real, but it’s small. It typically stays well below the 100.4°F threshold that defines a fever. Think of it as your body running a fraction warmer, not as being sick.
What Counts as a Real Fever
In pregnancy, a temperature at or above 100.4°F (38°C) is considered a fever. This distinction matters because the hormonal warmth of early pregnancy might push your resting temperature from, say, 97.8°F to 98.2°F, but it won’t push it into the 100s. If your thermometer reads 100.4°F or higher, something other than pregnancy hormones is going on, and the cause needs to be identified.
Common Causes of Actual Fever in Early Pregnancy
Pregnant women get sick for the same reasons everyone else does. The most frequent culprits behind a true fever in the first trimester include:
- Urinary tract infections (UTIs): Pregnancy increases UTI risk because hormonal changes slow urine flow and make bacteria more likely to settle in the bladder. A UTI can cause fever along with burning during urination, pelvic pressure, or cloudy urine.
- Respiratory infections: Colds, the flu, and COVID-19 all circulate regardless of pregnancy status, and your immune system undergoes subtle shifts in early pregnancy that can make you more susceptible.
- Gastrointestinal infections: Food poisoning or stomach viruses can cause fever alongside vomiting and diarrhea.
- Listeriosis: A bacterial infection from contaminated food (soft cheeses, deli meats, unpasteurized products) that is rare but poses particular risks during pregnancy.
A study of pregnant women who visited a gynecological emergency department with fevers of 100.4°F or higher found that infections were the predominant cause. The fever itself was not from the pregnancy.
Why Fever in the First Trimester Matters
A true fever during the weeks around conception and through the first trimester carries specific risks that make it worth taking seriously. The most studied concern involves neural tube defects, which are problems with how the brain and spinal cord form in the earliest weeks of development. A large study comparing 375 pregnancies affected by neural tube defects to over 8,200 unaffected pregnancies found that mothers who reported a fever in the month before or after their last menstrual period had a 2.4 times greater risk of a neural tube defect like spina bifida.
Folic acid intake made a significant difference. Among mothers who were not getting at least 400 micrograms of folic acid per day and who also had a fever during that critical window, the risk jumped to 4.2 times higher compared to mothers with adequate folic acid and no fever. For mothers who did take adequate folic acid, fever raised the risk only modestly, to about 1.8 times, and that increase was not statistically significant. This is one of several reasons prenatal vitamins with folic acid are recommended starting before conception.
It’s worth putting these numbers in perspective. Neural tube defects are uncommon overall, so even a doubled or tripled relative risk still translates to a small absolute chance. But reducing fever promptly and maintaining folic acid intake are two straightforward ways to lower that risk further.
Managing a Fever Safely
Acetaminophen (Tylenol) remains the safest over-the-counter option for bringing down a fever during pregnancy. Aspirin and ibuprofen (Advil, Motrin) have well-documented risks to fetal development, so they should be avoided. That said, the FDA has noted that some evidence links prolonged, chronic use of acetaminophen throughout pregnancy to a small increased risk of neurological conditions in children. The practical takeaway: using acetaminophen for a few days to treat an actual fever is reasonable, but taking it continuously for weeks without a clear medical need is not ideal.
Beyond medication, staying hydrated, resting, and using cool compresses can help bring your temperature down. If the fever is caused by a bacterial infection like a UTI, treating the underlying infection is the most important step.
Signs That Need Immediate Attention
The CDC lists a fever of 100.4°F or higher during pregnancy as an urgent maternal warning sign on its own. Certain combinations of symptoms raise the urgency further:
- Fever with severe belly pain that doesn’t go away, starts suddenly, or worsens over time
- Fever with nausea and vomiting so severe you cannot keep fluids down, especially if accompanied by dizziness, confusion, or dry mouth
- Fever with severe chest, shoulder, or back pain
A brief, low-grade temperature that resolves quickly is less concerning than a persistent or high fever. But in early pregnancy, even a moderate fever is worth a phone call to your provider so the cause can be identified and treated before it lingers.