Kidney infection (pyelonephritis) during pregnancy

By trimester

Weeks 1–13

1st trimester

Kidney infections are less common this early: only about 10 to 20 percent of pregnancy cases happen in the first trimester. Still, a bladder infection can climb before you notice more than mild urgency, so report any fever or back pain to your provider rather than waiting for your next visit.

Weeks 14–27

2nd trimester

This is one of the two trimesters where kidney infections happen most, as the ureters widen further under progesterone's effect and drain more slowly. New fever, chills, or pain below the ribs on one side is worth a same-day call.

Weeks 28–birth

3rd trimester

The growing uterus adds mechanical pressure on top of the hormonal slowing, and pyelonephritis now carries added weight, since a meaningful share of untreated cases have been linked to earlier delivery. Any fever with flank pain deserves prompt evaluation rather than a wait-and-see approach.

What does the evidence show for kidney infection?

• A kidney infection differs from an ordinary bladder infection by adding fever of 100.4°F or higher, flank pain, chills, and often nausea or vomiting on top of urinary symptoms like burning or urgency. 1
• Pregnancy raises the risk on its own: rising progesterone relaxes the muscle lining the ureters while the growing uterus presses against them, so urine drains more slowly and bacteria get more time to travel upward, part of why kidney infections cluster in the second and third trimesters, with only about 10 to 20 percent showing up in the first. 2
• In a three-year cohort of 158 pregnancies with a kidney infection, 13.3 percent developed sepsis, 11 percent delivered before 37 weeks, and 7.6 percent needed high-dependency or intensive unit care, which is why early recognition matters. 3
• For carefully selected, uncomplicated cases, a randomized trial found that starting antibiotics in the hospital and finishing the course at home was just as safe as staying admitted: only 12 percent of the outpatient group needed readmission, and none had an early delivery or other serious complication. 4
• A fever that doesn't come down despite antibiotics already underway can point to a resistant bacteria, a hidden abscess, or a kidney stone blocking drainage, all of which need further evaluation rather than simply extending the same antibiotics. 1

When should I call my provider about kidney infection?

Fever of 100.4°F or higher with flank or back pain, chills, or nausea and vomiting is the combination that separates a kidney infection from an ordinary bladder infection and calls for same-day evaluation. A racing heartbeat, low blood pressure, confusion, or new shortness of breath on top of those symptoms can signal the infection spreading into the bloodstream, which is an emergency. A fever that doesn't come down despite antibiotics already underway, or pain that keeps getting worse, is also worth an urgent call, since it can point to a resistant infection, an abscess, or a blocked kidney needing further evaluation. None of these lists replace your own judgment: if this symptom worries you, that on its own is enough reason to check in with your healthcare provider.