diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 9989812..777833e 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -25,6 +25,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select, func
 
 from app.database import get_db
+from app.services.connection import sync_birthday_to_contacts
 from app.models.user import User
 from app.models.session import UserSession
 from app.models.settings import Settings
@@ -686,6 +687,7 @@ async def update_profile(
         current_user.email = update_data["email"]
     if "date_of_birth" in update_data:
         current_user.date_of_birth = update_data["date_of_birth"]
+        await sync_birthday_to_contacts(db, current_user.id)
     if "umbral_name" in update_data:
         current_user.umbral_name = update_data["umbral_name"]
 
diff --git a/backend/app/routers/settings.py b/backend/app/routers/settings.py
index c99de7f..877a393 100644
--- a/backend/app/routers/settings.py
+++ b/backend/app/routers/settings.py
@@ -7,6 +7,7 @@ from app.models.settings import Settings
 from app.models.user import User
 from app.schemas.settings import SettingsUpdate, SettingsResponse
 from app.routers.auth import get_current_user, get_current_settings
+from app.services.connection import sync_birthday_to_contacts
 
 router = APIRouter()
 
@@ -94,6 +95,9 @@ async def update_settings(
     for key, value in update_data.items():
         setattr(current_settings, key, value)
 
+    if "share_birthday" in update_data:
+        await sync_birthday_to_contacts(db, current_settings.user_id)
+
     await db.commit()
     await db.refresh(current_settings)
 
diff --git a/backend/app/services/connection.py b/backend/app/services/connection.py
index 09cbbb9..270d487 100644
--- a/backend/app/services/connection.py
+++ b/backend/app/services/connection.py
@@ -9,6 +9,7 @@ from datetime import date as date_type
 from types import SimpleNamespace
 from typing import Optional
 
+from sqlalchemy import select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.person import Person
@@ -134,6 +135,25 @@ def create_person_from_connection(
     )
 
 
+
+async def sync_birthday_to_contacts(db: AsyncSession, user_id: int) -> None:
+    """Sync user's DOB to all Person records where linked_user_id == user_id.
+    Respects share_birthday setting — if disabled, clears birthday on linked records."""
+    user = await db.execute(select(User).where(User.id == user_id))
+    user_obj = user.scalar_one()
+    settings_result = await db.execute(select(Settings).where(Settings.user_id == user_id))
+    settings_obj = settings_result.scalar_one_or_none()
+
+    share = settings_obj.share_birthday if settings_obj else False
+    new_birthday = user_obj.date_of_birth if share else None
+
+    await db.execute(
+        update(Person)
+        .where(Person.linked_user_id == user_id)
+        .values(birthday=new_birthday)
+    )
+
+
 async def detach_umbral_contact(person: Person) -> None:
     """Convert an umbral contact back to a standard contact. Does NOT commit.