The flu during pregnancy

By trimester

Weeks 1–13

1st trimester

Flu risk is higher from the earliest weeks of pregnancy. A high fever from flu in the first trimester has been linked in some studies to a higher chance of certain birth defects, so managing fever with a provider-approved option matters as much as addressing the flu itself. Getting a flu shot is still worth it even if you're already partway through flu season, since major medical groups recommend it during any trimester.

Weeks 14–27

2nd trimester

Flu risk doesn't go down in the second trimester. Guidance looks at pregnancy as a whole, not just one trimester, as the reason for higher complication risk. This is a good time to catch up on your annual flu shot if you missed it earlier, since the CDC and ACOG both say it has a long safety record across all three trimesters.

Weeks 28–birth

3rd trimester

Late in pregnancy, a larger uterus presses on your lungs and your heart works harder, so the strain flu already puts on breathing and circulation can feel more intense. If you're due in fall or winter, getting your flu shot in July or August, instead of waiting for the usual September or October window, can extend protective antibodies to your newborn until they're old enough for their own vaccine.

What does the evidence show for the flu?

• Getting the flu during pregnancy carries a higher chance of serious complications, including breathing problems severe enough to need hospitalization, compared with getting the flu while not pregnant. 1
• Pregnancy makes it harder for the body to fight off infections, so a serious flu illness can raise the chance of pregnancy complications like preterm delivery. 2
• Starting antiviral medication promptly is recommended for anyone pregnant with confirmed or suspected flu, and a flu shot during pregnancy also protects the baby for several months after birth since antibodies pass through to them. 3
• Pregnancy, and the two weeks after delivery, is one of the groups at higher risk for flu complications, which is why starting antiviral medication early matters for confirmed or suspected flu. 4
• Antiviral medication works best when started within 48 hours of symptoms beginning, and a flu shot during pregnancy does double duty, since it passes protective antibodies to the baby until they can get their own vaccine at 6 months old. 5
• Acute influenza symptoms persist about 7 to 10 days and are self-limited in most healthy people, but a severe course can progress to shortness of breath, tachycardia, and hypotension in as little as 48 hours, and pregnancy is listed among the groups more prone to primary viral and secondary bacterial pneumonia. 6

When should I call my provider about the flu?

Flu usually runs 7 to 10 days and resolves on its own, so the signs worth acting on are the ones that mark a severe course: shortness of breath, a racing heart, and lightheadedness or a blood pressure drop. Severe influenza can build in as little as 48 hours from the first symptom, which is why waiting out the week is the wrong move in pregnancy. Feeling better and then getting worse again with a new fever is the shape of a secondary bacterial pneumonia, a complication pregnancy makes more likely. A high fever in the first trimester is its own reason to call the same day. If you are ever unsure which side of the line you are on, make the call. Obstetric teams handle these check-ins as routine, not as overreacting.