Heat rash during pregnancy

By trimester

Weeks 1–13

1st trimester

Early pregnancy hasn't yet brought the big rise in blood volume and metabolic heat that peaks later, so heat rash is less common this early unless you're dealing with hot weather or a fever.

Weeks 14–27

2nd trimester

As blood volume and sweat gland activity climb through the second trimester, heat rash becomes more likely, especially in skin folds, under the breasts, or anywhere clothing sits snugly.

Weeks 28–birth

3rd trimester

By the third trimester, added body weight, a warmer core temperature, and reduced heat tolerance make heat rash most likely, so loose, breathable fabric and staying cool matter more than ever.

What does the evidence show for heat rash?

• Pregnancy raises plasma volume and cardiac output by close to 50% by the third trimester, along with a lower sweating threshold and extra metabolic heat from the fetus and placenta, all of which add up to more sweat production. 1
• Eccrine sweat gland activity increases during pregnancy while apocrine gland activity decreases, a shift that researchers note may contribute to skin symptoms in this group. 2
• Physiologic skin changes of some kind show up in the large majority of pregnancies, and glandular changes like these are considered a normal, generally benign, self-limiting part of pregnancy that resolves after delivery. 3
• Heat rash, or miliaria rubra, typically appears as small red, non-follicular bumps or fluid-filled spots, and while it's usually mild, secondary bacterial infection is a recognized complication that can call for antimicrobial care. 4
• Even with pregnancy's built-in heat adaptations, researchers note the body's protective mechanisms could be overwhelmed during extreme heat exposure, part of why staying cool matters more than it might otherwise. 1

When should I call my provider about heat rash?

Heat rash itself is a benign, common skin reaction, but bumps that spread, ooze, or turn more painful than itchy, especially alongside fever, point toward a secondary bacterial infection rather than simple heat rash. That combination is a reason to check in with your provider rather than waiting it out, since a skin infection is different from ordinary blocked sweat ducts and may call for antimicrobial care. A rash that doesn't ease within about a week of consistent cooling is also worth a second look. 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.