diff --git a/src/intelstream/services/pipeline.py b/src/intelstream/services/pipeline.py
index 32abfee..4180208 100644
--- a/src/intelstream/services/pipeline.py
+++ b/src/intelstream/services/pipeline.py
@@ -242,12 +242,20 @@ async def _fetch_source(self, source: Source) -> int:
                     logger.debug("Content item already exists", external_id=item.external_id)
                     continue
 
-                if is_first_poll:
+        if is_first_poll and new_count > 0:
+            most_recent = await self._repository.get_most_recent_item_for_source(source.id)
+            if most_recent:
+                backfilled = await self._repository.mark_items_as_backfilled(
+                    source_id=source.id,
+                    exclude_item_id=most_recent.id,
+                )
+                if backfilled > 0:
                     logger.info(
-                        "First poll for source, limiting to most recent item",
+                        "First poll: backfilled pre-existing items",
                         source_name=source.name,
+                        backfilled_count=backfilled,
+                        most_recent_title=most_recent.title,
                     )
-                    break
 
         await self._repository.update_source_last_polled(source.id)
 
diff --git a/tests/test_services/test_pipeline.py b/tests/test_services/test_pipeline.py
index 4e90f32..c4dee40 100644
--- a/tests/test_services/test_pipeline.py
+++ b/tests/test_services/test_pipeline.py
@@ -176,6 +176,10 @@ async def test_fetch_all_sources_success(
 
         mock_repository.get_all_sources.return_value = [sample_source]
         mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = MagicMock(
+            spec=ContentItem, id="item-1", title="Test"
+        )
+        mock_repository.mark_items_as_backfilled.return_value = 0
 
         with patch.object(
             pipeline._adapters[SourceType.SUBSTACK],
@@ -253,17 +257,21 @@ async def test_fetch_all_sources_no_adapter_for_type(
 
         await pipeline.close()
 
-    async def test_first_poll_limits_to_one_item(
+    async def test_first_poll_stores_all_items_and_backfills(
         self,
         pipeline: ContentPipeline,
         mock_repository: AsyncMock,
         sample_source,
     ):
-        """On first poll (last_polled_at is None), only store the most recent item."""
+        """On first poll, store all items but backfill all except the most recent."""
         await pipeline.initialize()
 
         sample_source.last_polled_at = None
 
+        most_recent = MagicMock(spec=ContentItem)
+        most_recent.id = "most-recent-id"
+        most_recent.title = "Article 4"
+
         items = [
             ContentData(
                 external_id=f"article-{i}",
@@ -279,6 +287,8 @@ async def test_first_poll_limits_to_one_item(
 
         mock_repository.get_all_sources.return_value = [sample_source]
         mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = most_recent
+        mock_repository.mark_items_as_backfilled.return_value = 4
 
         with patch.object(
             pipeline._adapters[SourceType.SUBSTACK],
@@ -288,8 +298,48 @@ async def test_first_poll_limits_to_one_item(
         ):
             result = await pipeline.fetch_all_sources()
 
+        assert result == 5
+        assert mock_repository.add_content_item.call_count == 5
+        mock_repository.get_most_recent_item_for_source.assert_called_once_with(sample_source.id)
+        mock_repository.mark_items_as_backfilled.assert_called_once_with(
+            source_id=sample_source.id,
+            exclude_item_id=most_recent.id,
+        )
+
+        await pipeline.close()
+
+    async def test_first_poll_no_backfill_when_single_item(
+        self,
+        pipeline: ContentPipeline,
+        mock_repository: AsyncMock,
+        sample_source,
+        sample_content_data,
+    ):
+        """On first poll with a single item, backfill marks 0 items."""
+        await pipeline.initialize()
+
+        sample_source.last_polled_at = None
+
+        most_recent = MagicMock(spec=ContentItem)
+        most_recent.id = "item-id"
+        most_recent.title = "Test Article"
+
+        mock_repository.get_all_sources.return_value = [sample_source]
+        mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = most_recent
+        mock_repository.mark_items_as_backfilled.return_value = 0
+
+        with patch.object(
+            pipeline._adapters[SourceType.SUBSTACK],
+            "fetch_latest",
+            new_callable=AsyncMock,
+            return_value=[sample_content_data],
+        ):
+            result = await pipeline.fetch_all_sources()
+
         assert result == 1
         assert mock_repository.add_content_item.call_count == 1
+        mock_repository.mark_items_as_backfilled.assert_called_once()
 
         await pipeline.close()
 
@@ -518,6 +568,10 @@ async def test_fetch_success_resets_failure_count(
 
         mock_repository.get_all_sources.return_value = [sample_source]
         mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = MagicMock(
+            spec=ContentItem, id="item-1", title="Test"
+        )
+        mock_repository.mark_items_as_backfilled.return_value = 0
 
         with patch.object(
             pipeline._adapters[SourceType.SUBSTACK],
@@ -545,6 +599,10 @@ async def test_fetch_passes_skip_content_to_adapter(
 
         mock_repository.get_all_sources.return_value = [sample_source]
         mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = MagicMock(
+            spec=ContentItem, id="item-1", title="Test"
+        )
+        mock_repository.mark_items_as_backfilled.return_value = 0
 
         with patch.object(
             pipeline._adapters[SourceType.SUBSTACK],
@@ -627,6 +685,10 @@ async def test_never_skips_first_poll(
         sample_source.last_polled_at = None
         mock_repository.get_all_sources.return_value = [sample_source]
         mock_repository.content_item_exists.return_value = False
+        mock_repository.get_most_recent_item_for_source.return_value = MagicMock(
+            spec=ContentItem, id="item-1", title="Test"
+        )
+        mock_repository.mark_items_as_backfilled.return_value = 0
 
         with patch.object(
             pipeline._adapters[SourceType.SUBSTACK],