重新发送后获取NULL值

Question

Good day community I'm having some trouble with my code i have a textbox with PHP value.

<label for="">Surname</label>
<input class="form-control"type="text" name="pasurname" id="pasurname" value="<?php echo $row['pasurname']; ?>">

But after re-sending it with its new value. i keep getting null. or blank

if(isset($_POST['viewprenataldataid']))  {  
  $pasurname = $_POST['pasurname'];
  $viewprenataldataid = $_POST['viewprenataldataid'];
  $query  =  mysqli_query($con,"UPDATE prenatalform SET pasurname='$pasurname' WHERE id = '$viewprenataldataid'");  

}

And here is my sample ajax

        $('#update').click(function(){
            $.ajax({  
                 url:"updateprenatal.php",  
                 method:"POST",  
                 data:$('#updateprenatalform').serialize(),  
                 beforeSend:function(){  
                      $('#update').val("Updating");
                 },  
                 success:function(data){  
                      $('form').trigger("reset"); 

                        alert(data);
                 }  
            });  
        }); 

I always getting error from alert data that $_POST['pasurname'] is undefined index. but after an hour of checking it. im pretty sure that it is correct. by the way my textbox are also came from a ajax. using append(data)