# Day 145: Fever at 4 to 5 months

> 100.4°F is a fever. At 3 to 6 months that means a call within 24 hours, sooner for 104°F or a baby who seems very sick. The thermometer rules, the medicine rules, and a practice run on a well day.

- Series: Your fifth month, day by day (https://chorevia.com/learn/fifth-month), day 145 of 365, week 20
- Canonical URL: https://chorevia.com/learn/fifth-month/day-145
- Last reviewed: 2026-10-21
- Publisher: Chorevia (https://chorevia.com)
- Cite as: Chorevia. "Day 145: Fever at 4 to 5 months." Last reviewed October 21, 2026. https://chorevia.com/learn/fifth-month/day-145

This guide is general education that summarizes published research and guidance from groups like the American Academy of Pediatrics, the CDC, and the WHO. It isn’t medical advice, and every child develops on their own timeline. Your pediatrician knows your child; if something feels wrong, call them. In an emergency, call 911.

Nobody wants their first try at a baby's temperature to be at 3 a.m. with a thermometer still in the box. Today is the practice run, on a well day.

## Heart

A rectal temperature of 100.4°F (38°C) or higher is a fever. For babies 3 to 6 months, the AAP advises contacting your pediatrician within 24 hours about any fever, sooner for 104°F (40°C) or higher or a baby who seems very sick ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)); its fever article singles out 101°F (38.3°C) or higher as a reason to call ([AAP](https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx)). Don't use ear thermometers before 6 months ([AAP](https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx)).

## Head

The medicine rules are strict at this age. The AAP says no acetaminophen under 2 without a doctor's guidance ([AAP](https://www.healthychildren.org/English/safety-prevention/at-home/medication-safety/Pages/Acetaminophen-for-Fever-and-Pain.aspx)) and no ibuprofen before 6 months unless your doctor says to ([AAP](https://www.healthychildren.org/English/safety-prevention/at-home/medication-safety/Pages/Ibuprofen-for-Fever-and-Pain.aspx)). Under 6 months, the only fluids are breast milk or formula ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Immunization+Reactions)). And "it's just teeth" isn't an answer: in a study of 125 babies, fevers over 102°F weren't linked to teething ([Macknin et al., 2000](https://pubmed.ncbi.nlm.nih.gov/10742315/)), and a meta-analysis of 16 studies agreed that any temperature rise with teething didn't amount to a fever ([Massignan et al., 2016](https://pubmed.ncbi.nlm.nih.gov/26908659/)).

## Body

When you call, the office will want three things: the number, how you took it, and how your baby is acting. That last one matters on its own; looking or acting very sick, or your sense that your baby needs to be seen urgently, is a reason to call right away whatever the thermometer says ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)). The [fever section](https://chorevia.com/learn/4-months#fever-at-4-to-5-months) of the 4-month guide has the full call-now and ER lists in one place.

## Try today

### The temperature practice

- **Time:** 5 minutes, with the baby fed and calm
- **From the guide:** [Fever at 4 to 5 months](https://chorevia.com/learn/4-months#fever-at-4-to-5-months)

1. Get out the digital thermometer, not an ear one ([AAP](https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx)), and read its instructions for a rectal reading; if anything is unclear, ask the office's nurse line to talk you through it.
2. Lay your baby on their back on the floor mat, one hand on them, and take the reading the way the instructions and your pediatrician's office showed you, talking the whole time.
3. Write the number and the time in the notebook, so you know your baby's normal before you need to know abnormal.
4. Put the thermometer, the office number, and one line, "100.4°F or higher: call within 24 hours; 104°F or very sick: call now," on a card in the changing basket ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)).

**Make it work:** A practice reading proves nothing about your baby's health. It makes the real one calmer.

## Watch for

- **Normal:** a warm head after a nap, a cry, or a hat. A rectal temperature of 100.4°F (38°C) or higher is a fever, and at 3 to 6 months any fever means **contact your pediatrician within 24 hours** ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)).
- **Normal:** a slight temperature rise in the days around a new tooth, which isn't a fever ([Macknin et al., 2000](https://pubmed.ncbi.nlm.nih.gov/10742315/)). **Call your pediatrician right away** for 104°F (40°C) or higher, nonstop crying, or a baby who cries when touched or moved ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)).
- **Normal:** a sleepier baby with a mild cold. **Go to the ER now** for a bulging soft spot, a baby who is hard to wake, or one who isn't alert when awake, and **call 911** for purple or blood-colored spots on the skin with a fever, or a baby who can't be woken up ([AAP](https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever)).

## For you

Your own fever is still an urgent warning sign, twenty weeks out. A fever of 100.4°F or higher is on the CDC's list of urgent maternal warning signs, the kind to seek care for immediately, and the list applies for a year after birth; tell any provider you were pregnant within the last year ([CDC](https://www.cdc.gov/hearher/maternal-warning-signs/index.html)). The discharge instructions say a fever above 100°F that persists is a call ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000628.htm)).

If you're breastfeeding, the one to know is a tender, red, warm area on one breast, which is also a call ([MedlinePlus](https://medlineplus.gov/ency/patientinstructions/000624.htm)). Flu-ish with a sore spot isn't something to feed through for three days.

If you're the partner: the thermometer works on her too, and so does the card. If she's hot and miserable, you make the call.

## Sources

- American Academy of Pediatrics. "Fever" (symptom checker). HealthyChildren.org, 2026. https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Fever
- American Academy of Pediatrics. "Fever and Your Baby." HealthyChildren.org, 2025. https://www.healthychildren.org/English/health-issues/conditions/fever/Pages/Fever-and-Your-Baby.aspx
- American Academy of Pediatrics. "Acetaminophen Dosing Tables for Fever and Pain in Children." HealthyChildren.org, 2025. https://www.healthychildren.org/English/safety-prevention/at-home/medication-safety/Pages/Acetaminophen-for-Fever-and-Pain.aspx
- American Academy of Pediatrics. "Ibuprofen Dosing Table for Fever and Pain." HealthyChildren.org, 2024. https://www.healthychildren.org/English/safety-prevention/at-home/medication-safety/Pages/Ibuprofen-for-Fever-and-Pain.aspx
- American Academy of Pediatrics. "Immunization Reactions" (symptom checker). HealthyChildren.org, 2026. https://www.healthychildren.org/English/tips-tools/symptom-checker/Pages/symptomviewer.aspx?symptom=Immunization+Reactions
- Macknin ML, Piedmonte M, Jacobs J, Skibinski C. "Symptoms associated with infant teething: a prospective study." Pediatrics, 2000. https://pubmed.ncbi.nlm.nih.gov/10742315/
- Massignan C, Cardoso M, Porporatti AL, et al. "Signs and Symptoms of Primary Tooth Eruption: A Meta-analysis." Pediatrics, 2016. https://pubmed.ncbi.nlm.nih.gov/26908659/
- Centers for Disease Control and Prevention. "Urgent Maternal Warning Signs and Symptoms." HEAR HER, 2024. https://www.cdc.gov/hearher/maternal-warning-signs/index.html
- MedlinePlus. "Vaginal delivery - discharge." National Library of Medicine, 2024. https://medlineplus.gov/ency/patientinstructions/000628.htm
- MedlinePlus. "Going home after a C-section." National Library of Medicine, 2024. https://medlineplus.gov/ency/patientinstructions/000624.htm