diff --git a/nbs/05_pheno_loader.ipynb b/nbs/05_pheno_loader.ipynb
index 925e4ac..e217c53 100644
--- a/nbs/05_pheno_loader.ipynb
+++ b/nbs/05_pheno_loader.ipynb
@@ -518,7 +518,9 @@
     "                continue\n",
     "            \n",
     "            if table_name == 'age_sex':\n",
-    "                keep_undefined = True\n",
+    "                # The 'age_sex' table does not contain 'undefined', so the merge will not cause a Cartesian product\n",
+    "                keep_undefined = True \n",
+    "                # Left join to keep only rows with real data points\n",
     "                how = 'left'\n",
     "            else: \n",
     "                keep_undefined = keep_undefined_research_stage\n",
@@ -581,13 +583,13 @@
     "        return False\n",
     "    \n",
     "    @staticmethod\n",
-    "    def join_and_filter_undefined_research_stage(df1, df2, how='outer'):\n",
+    "    def join_and_filter_undefined_research_stage(df1, df2, how='outer', lsuffix='', rsuffix=''):\n",
     "        df1_defined = df1[df1.index.get_level_values('research_stage') != 'undefined']\n",
     "        df2_defined = df2[df2.index.get_level_values('research_stage') != 'undefined']\n",
     "\n",
-    "        return df1_defined.join(df2_defined, how=how)\n",
+    "        return df1_defined.join(df2_defined, how=how, lsuffix=lsuffix, rsuffix=rsuffix)\n",
     "\n",
-    "    def __concat__(self, df1, df2, keep_undefined_research_stage=False, how='outer'):\n",
+    "    def __concat__(self, df1, df2, keep_undefined_research_stage=False, how='outer', lsuffix='', rsuffix=''):\n",
     "\n",
     "        if df1.empty:\n",
     "            return df2\n",
@@ -598,10 +600,10 @@
     "            self.is_value_in_index(df2, 'undefined', 'research_stage') and not keep_undefined_research_stage:\n",
     "        \n",
     "            warnings.warn('filtering \"undefined\" research_stage..')\n",
-    "            df = self.join_and_filter_undefined_research_stage(df1, df2, how)\n",
+    "            df = self.join_and_filter_undefined_research_stage(df1, df2, how, lsuffix='', rsuffix='')\n",
     "            return df\n",
     "        \n",
-    "        return df1.join(df2, how=how)\n",
+    "        return df1.join(df2, how=how, lsuffix=lsuffix, rsuffix=rsuffix)\n",
     "    \n",
     "    def merge_all_tables(self) -> pd.DataFrame:\n",
     "        # merge all tables in self.dfs dictionary\n",
@@ -610,8 +612,8 @@
     "            if align_df is None:\n",
     "                align_df = df\n",
     "            else:\n",
-    "                align_df = pd.merge(align_df, df, left_index=True, right_index=True, how='outer', suffixes=('', name))\n",
-    "                \n",
+    "                # Join the table with an 'undefined' research_stage to keep the maximum number of data points\n",
+    "                align_df = self.__concat__(align_df, df, keep_undefined_research_stage=True, how='outer', lsuffix='',  rsuffix= name)                \n",
     "        return align_df\n",
     "\n",
     "    def __load_age_sex__(self) -> None:\n",
diff --git a/pheno_utils/pheno_loader.py b/pheno_utils/pheno_loader.py
index f302792..b4f0528 100644
--- a/pheno_utils/pheno_loader.py
+++ b/pheno_utils/pheno_loader.py
@@ -467,7 +467,9 @@ def get(self, fields: Union[str,List[str]], flexible: bool=None, not_bulk_field=
                 continue
             
             if table_name == 'age_sex':
-                keep_undefined = True
+                # The 'age_sex' table does not contain 'undefined', so the merge will not cause a Cartesian product
+                keep_undefined = True 
+                # Left join to keep only rows with real data points
                 how = 'left'
             else: 
                 keep_undefined = keep_undefined_research_stage
@@ -530,13 +532,13 @@ def is_value_in_index(self, df, value, index_name):
         return False
     
     @staticmethod
-    def join_and_filter_undefined_research_stage(df1, df2, how='outer'):
+    def join_and_filter_undefined_research_stage(df1, df2, how='outer', lsuffix='', rsuffix=''):
         df1_defined = df1[df1.index.get_level_values('research_stage') != 'undefined']
         df2_defined = df2[df2.index.get_level_values('research_stage') != 'undefined']
 
-        return df1_defined.join(df2_defined, how=how)
+        return df1_defined.join(df2_defined, how=how, lsuffix=lsuffix, rsuffix=rsuffix)
 
-    def __concat__(self, df1, df2, keep_undefined_research_stage=False, how='outer'):
+    def __concat__(self, df1, df2, keep_undefined_research_stage=False, how='outer', lsuffix='', rsuffix=''):
 
         if df1.empty:
             return df2
@@ -547,10 +549,10 @@ def __concat__(self, df1, df2, keep_undefined_research_stage=False, how='outer')
             self.is_value_in_index(df2, 'undefined', 'research_stage') and not keep_undefined_research_stage:
         
             warnings.warn('filtering "undefined" research_stage..')
-            df = self.join_and_filter_undefined_research_stage(df1, df2, how)
+            df = self.join_and_filter_undefined_research_stage(df1, df2, how, lsuffix='', rsuffix='')
             return df
         
-        return df1.join(df2, how=how)
+        return df1.join(df2, how=how, lsuffix=lsuffix, rsuffix=rsuffix)
     
     def merge_all_tables(self) -> pd.DataFrame:
         # merge all tables in self.dfs dictionary
@@ -559,8 +561,8 @@ def merge_all_tables(self) -> pd.DataFrame:
             if align_df is None:
                 align_df = df
             else:
-                align_df = pd.merge(align_df, df, left_index=True, right_index=True, how='outer', suffixes=('', name))
-                
+                # Join the table with an 'undefined' research_stage to keep the maximum number of data points
+                align_df = self.__concat__(align_df, df, keep_undefined_research_stage=True, how='outer', lsuffix='',  rsuffix= name)                
         return align_df
 
     def __load_age_sex__(self) -> None: