Flu During Pregnancy — Increased Risks, Safe Treatments, and Warning Signs
How pregnancy increases flu severity and complication risk, which treatments are safe during each trimester, essential warning signs, and why the flu vaccine is critical.
Medically reviewed by
Dr. James Chen, PhD, MPH
Pregnancy changes nearly every system in the body — including how the body responds to influenza. What might be a manageable week of illness for a non-pregnant adult can become a serious, even life-threatening, medical event during pregnancy. The CDC, WHO, and NHS all classify pregnant people as a high-priority group for influenza prevention and early treatment. Understanding the specific risks, safe treatments, and warning signs is essential for every pregnant person and their healthcare team.
- Pregnant people are more than twice as likely to be hospitalized with influenza compared to non-pregnant people of the same age (CDC).
- The risk increases with each trimester and is highest in the third trimester and postpartum period (first 2 weeks after delivery).
- Flu during pregnancy is associated with increased risk of preterm birth, low birth weight, and fetal distress.
- The flu vaccine is safe and recommended during any trimester of pregnancy.
Why Pregnancy Increases Flu Severity
Pregnancy induces profound physiologic and immunologic changes that collectively increase susceptibility to severe influenza:
1. Immune System Modulation
Pregnancy involves a carefully calibrated shift in immune function. To tolerate the developing fetus (which is genetically half foreign), the body downregulates certain T-cell responses (Th1) and upregulates others (Th2). This shift, while essential for maintaining pregnancy, reduces the ability to fight intracellular pathogens like influenza virus. The immune system is not "weakened" in a simple sense — it is intentionally modulated — but the net effect is a reduced antiviral defense.
2. Cardiovascular and Respiratory Changes
By the third trimester, a pregnant person's heart rate increases by 15–20%, blood volume expands by 40–50%, and oxygen consumption increases by 20%. The growing uterus pushes the diaphragm upward, reducing lung capacity by 10–25%. These changes leave less respiratory reserve, so an infection that reduces lung function can tip a pregnant person into respiratory failure faster than a non-pregnant person.
3. Increased Metabolic Demands
Fever and the increased metabolic demands of fighting infection can lead to dehydration and metabolic stress more quickly during pregnancy. The combination of reduced oral intake (from nausea, poor appetite) and increased fluid losses (from fever, sweating) can cause hypovolemia that compromises placental blood flow.
4. Hypercoagulable State
Pregnancy is a pro-thrombotic state — the blood clots more easily. Severe influenza adds further inflammatory stress to the coagulation system, potentially increasing the risk of thromboembolic events.
How Flu Symptoms Present During Pregnancy
The symptoms of influenza during pregnancy are similar to those in non-pregnant adults, but with important nuances:
| Symptom | Pregnancy Considerations |
|---|---|
| Fever | Particularly concerning in first trimester — hyperthermia is associated with increased risk of neural tube defects. Fever should be treated promptly with acetaminophen. |
| Shortness of breath | Key distinction: "normal" pregnancy shortness of breath is mild and chronic; acute or worsening shortness of breath with flu symptoms is a red flag. |
| Cough | Persistent coughing with the gravid uterus can cause significant discomfort and abdominal muscle strain. |
| Body aches | May be difficult to distinguish from normal pregnancy musculoskeletal discomfort. |
| Nausea / vomiting | May be confused with morning sickness; acute worsening of nausea in later pregnancy warrants evaluation. |
| Reduced fetal movement | Not a direct flu symptom but a critical warning sign. Decreased fetal movement during maternal illness requires immediate evaluation. |
Safe Treatments During Pregnancy
Generally Considered Safe
| Medication | Use | Notes |
|---|---|---|
| Acetaminophen (Paracetamol) | Fever, pain, body aches | First-line choice; use lowest effective dose for shortest duration |
| Oseltamivir (Tamiflu) | Antiviral treatment | Recommended by CDC for pregnant people with suspected/confirmed flu; benefits outweigh risks |
| Saline nasal spray/drops | Nasal congestion | No systemic absorption; safe throughout pregnancy |
| Dextromethorphan (cough suppressant) | Dry cough | Generally considered safe after first trimester; consult provider |
| Guaifenesin (expectorant) | Productive cough | Generally considered safe; consult provider |
Medications to Avoid or Use With Caution
| Medication | Reason |
|---|---|
| NSAIDs (Ibuprofen, Naproxen, Aspirin) | Associated with oligohydramnios (low amniotic fluid), premature closure of ductus arteriosus (especially third trimester), and increased bleeding risk. Avoid unless specifically prescribed. |
| Oral decongestants (Pseudoephedrine, Phenylephrine) | Vasoconstriction may reduce placental blood flow. Avoid in first trimester; limited use may be acceptable later with provider guidance. |
| Combination cold/flu products | May contain NSAIDs, decongestants, or alcohol. Read all labels carefully. |
| Codeine or other opioids | Neonatal withdrawal risk; generally avoided. |
Warning Signs: When to Seek Emergency Care
- Difficulty breathing or shortness of breath at rest
- Severe chest pain or pressure
- Confusion or difficulty staying awake
- Persistent high fever (≥ 102°F / 38.9°C) not responding to acetaminophen
- Decreased or absent fetal movement
- Vaginal bleeding or fluid leakage
- Regular contractions or abdominal pain suggesting preterm labor
- Severe vomiting preventing oral intake of fluids
- Signs of dehydration (dizziness when standing, very dark urine, racing heart)
- You have suspected flu and you are in a high-risk pregnancy (multiple gestation, pre-existing conditions)
- Fever persists more than 48 hours despite treatment
- You notice any change in fetal movement patterns
- You cannot keep fluids down for 12+ hours
- Symptoms improve and then worsen (possible secondary bacterial infection)
Antiviral Treatment: Don't Delay
The CDC explicitly recommends that pregnant people with suspected or confirmed influenza receive prompt antiviral treatment — ideally within 48 hours of symptom onset. Oseltamivir (Tamiflu) is the preferred antiviral in pregnancy due to the largest body of safety data. Key points:
- Do not wait for test results to start treatment if flu is clinically suspected
- Treatment is recommended regardless of gestational age (any trimester)
- The benefits of treatment (reduced severity, reduced hospitalization, reduced obstetric complications) outweigh the small theoretical risks
- Treatment may still provide benefit even when started after 48 hours in severely ill or hospitalized patients
The Flu Vaccine in Pregnancy: Safety and Importance
The inactivated influenza vaccine is one of the most extensively studied vaccines in pregnancy and has a strong safety record. The CDC, WHO, NHS, and ACOG all recommend it at any stage of pregnancy:
- Protects the pregnant person: Reduces the risk of flu-related hospitalization by an estimated 40% (CDC).
- Protects the baby: Maternal antibodies cross the placenta and persist for several months after birth, protecting the newborn during the vulnerable period before they can be vaccinated at 6 months.
- Reduces adverse outcomes: Associated with lower rates of preterm birth, low birth weight, and stillbirth during flu season.
- Safe in any trimester: No evidence of harm to the fetus at any stage of pregnancy.
- The nasal spray vaccine (LAIV) should NOT be used during pregnancy — only the injectable inactivated vaccine.
Postpartum Considerations
The elevated risk does not end at delivery. The CDC notes that the first two weeks postpartum carry continued elevated risk. Breastfeeding people with influenza should:
- Continue breastfeeding — breast milk contains protective antibodies and is not a transmission route for influenza
- Practice careful hand hygiene and consider wearing a mask while feeding to reduce droplet transmission to the infant
- If too ill to breastfeed directly, expressed milk can be fed by another caregiver
- Antivirals (oseltamivir) are compatible with breastfeeding
References
- CDC — Pregnant Women & Influenza (Flu)
- CDC — Treatment of Influenza During Pregnancy
- NHS — Flu vaccine in pregnancy
- WHO — Influenza (Seasonal)
- ACOG — Influenza Vaccination During Pregnancy
- Mertz D, et al. Pregnancy as a risk factor for severe outcomes from influenza. BMJ (2013).
- Siston AM, et al. Pandemic 2009 Influenza A(H1N1) Virus Illness Among Pregnant Women. JAMA (2010).
Sources
This article references information from the CDC, WHO, NHS, and peer-reviewed medical literature. Content is reviewed regularly for accuracy. Learn about our editorial policy.