diff --git a/modules/gptPreAuction.js b/modules/gptPreAuction.js
index ee2b5406453..c351a5181a9 100644
--- a/modules/gptPreAuction.js
+++ b/modules/gptPreAuction.js
@@ -33,11 +33,21 @@ export const appendGptSlots = adUnits => {
       const context = adUnit.ortb2Imp.ext.data;
       context.adserver = context.adserver || {};
       context.adserver.name = 'gam';
-      context.adserver.adslot = slot.getAdUnitPath();
+      context.adserver.adslot = sanitizeSlotPath(slot.getAdUnitPath());
     }
   });
 };
 
+const sanitizeSlotPath = (path) => {
+  const gptConfig = config.getConfig('gptPreAuction') || {};
+
+  if (gptConfig.mcmEnabled) {
+    return path.replace(/(^\/\d*),\d*\//, '$1/');
+  }
+
+  return path;
+}
+
 export const appendPbAdSlot = adUnit => {
   adUnit.ortb2Imp = adUnit.ortb2Imp || {};
   adUnit.ortb2Imp.ext = adUnit.ortb2Imp.ext || {};
diff --git a/test/spec/modules/gptPreAuction_spec.js b/test/spec/modules/gptPreAuction_spec.js
index c4a81c21d5c..3e8dbfe8d92 100644
--- a/test/spec/modules/gptPreAuction_spec.js
+++ b/test/spec/modules/gptPreAuction_spec.js
@@ -95,6 +95,31 @@ describe('GPT pre-auction module', () => {
       expect(adUnit.ortb2Imp.ext.data.adserver).to.deep.equal({ name: 'gam', adslot: 'slotCode2' });
     });
 
+    it('will trim child id if mcmEnabled is set to true', () => {
+      config.setConfig({ gptPreAuction: { enabled: true, mcmEnabled: true } });
+      window.googletag.pubads().setSlots([
+        makeSlot({ code: '/12345,21212/slotCode1', divId: 'div1' }),
+        makeSlot({ code: '/12345,21212/slotCode2', divId: 'div2' }),
+        makeSlot({ code: '/12345,21212/slotCode3', divId: 'div3' })
+      ]);
+      const adUnit = { code: '/12345,21212/slotCode2', ortb2Imp: { ext: { data: {} } } };
+      appendGptSlots([adUnit]);
+      expect(adUnit.ortb2Imp.ext.data.adserver).to.be.an('object');
+      expect(adUnit.ortb2Imp.ext.data.adserver).to.deep.equal({ name: 'gam', adslot: '/12345/slotCode2' });
+    });
+
+    it('will not trim child id if mcmEnabled is not set to true', () => {
+      window.googletag.pubads().setSlots([
+        makeSlot({ code: '/12345,21212/slotCode1', divId: 'div1' }),
+        makeSlot({ code: '/12345,21212/slotCode2', divId: 'div2' }),
+        makeSlot({ code: '/12345,21212/slotCode3', divId: 'div3' })
+      ]);
+      const adUnit = { code: '/12345,21212/slotCode2', ortb2Imp: { ext: { data: {} } } };
+      appendGptSlots([adUnit]);
+      expect(adUnit.ortb2Imp.ext.data.adserver).to.be.an('object');
+      expect(adUnit.ortb2Imp.ext.data.adserver).to.deep.equal({ name: 'gam', adslot: '/12345,21212/slotCode2' });
+    });
+
     it('should use the customGptSlotMatching function if one is given', () => {
       config.setConfig({
         gptPreAuction: {