What Do Petechiae Look Like in FNAIT Newborns?

Petechiae are a key clinical sign of low platelets, often representing the first visible clue of Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT) ^[1]. Because newborn skin is very sensitive and prone to benign (harmless) breakouts, parents need to know exactly how to differentiate a serious symptom from a normal rash.

What Do Petechiae Look Like?

To the naked eye, petechiae look like tiny, pinpoint dots that are red, purple, or brown ^[2]. They often look as though someone lightly tapped the baby’s skin with a fine-tip marker. Unlike common newborn skin issues, petechiae are completely flat and smooth to the touch—they are not raised bumps or pimples.

In a newborn with FNAIT, these spots can appear anywhere on the body, but they are frequently noticed on the torso (chest and belly) and groin ^[2]. They may also cluster along pressure points, such as where a diaper rubs or where the baby was gripped during delivery. (Note: Petechiae found only on the face or neck immediately after birth can sometimes be a harmless result of mechanical pressure during a difficult delivery, but they still require a doctor’s confirmation.)

The “Glass Test” (Blanching Test)

Newborns frequently develop harmless skin conditions like erythema toxicum (a common newborn rash) or baby acne. The most reliable way for a parent to tell a normal rash from petechiae at home is the glass test—known in the medical field as diascopy ^[3][4].

When you apply firm pressure to the skin:

• Normal rashes will “blanch”: The red spots will temporarily turn white or disappear ^[3][5]. This happens because the redness in a normal rash comes from swollen blood vessels; pressing pushes the blood out of the way.
• Petechiae are “non-blanching”: The dots will not fade, turn white, or change color when pressed ^[3][6]. Because the blood making up a petechia has already leaked out of the vessels and is trapped in the skin tissue, pressing on it cannot move the blood away.

How to do the glass test:

1. Find a clear, thick glass or a clear, hard plastic cup (avoid thin glass that could break under pressure).
2. Press the side or bottom of the cup firmly against the baby’s spots.
3. Look straight through the plastic or glass at the skin to see if the red spots disappear (blanch) or stay visible (non-blanching).

Other Signs to Watch For

If you are monitoring your baby for signs of low platelets, petechiae are not the only symptom. Be on the lookout for:

• Unexplained bruising (ecchymosis)
• Oozing or bleeding from the umbilical cord stump
• Bleeding from mucus membranes (like inside the mouth)
• Extreme lethargy, poor feeding, or unusual irritability

When to Seek Immediate Care

If your newborn has non-blanching spots (petechiae) or any other signs of bleeding, it requires an immediate trip to the Emergency Room or urgent pediatric care. Do not wait for a scheduled pediatrician appointment. When you arrive, immediately tell the triage nurse that your baby has (or is at risk for) FNAIT and may have critically low platelets. Fast action allows doctors to check platelet levels and provide treatments that prevent severe complications, including internal bleeding ^[7][1].