what is happening?

I think several things must be taken into account to optimize your code:

• Evaluate correctly what is happening at each moment, writing a controlled code
• Make error handling as specific as possible. In the case of PDO, errors can be obtained relative to the object PDO itself, or object PDOStatement . I think that here it is convenient to use the latter, to be more specific.
• You can also use rowCount() to show the number of rows that were inserted.

I would propose this solution:

$sql="INSERT INTO expediente (id_exp, peso, edad, estatura, fecha_registro, sexo, objetivo ) VALUES (:idx, :pes, :edd, :est, :fech, :sex, :obj)";

$stmt=$base->prepare($sql);
$arrParams=array(":idx"=>$id_exp, ":pes"=>$peso, ":edd"=>$edad, ":est"=>$estatura, ":fech"=>$fecha_registro, ":sex"=>$sexo, ":obj"=>$objeto);
$resultado=$stmt->execute($arrParams); 

$msgInfo=($resultado) ? "Se insertaron ".$stmt->rowCount(). " registros" : "¡Error! ".$stmt->errorInfo()[2];
echo $msgInfo;

The key here is the line where the variable $msgInfo is created. It evaluates the value of $resultado through a ternary operator. If it is true, it means that the query occurred successfully, in that case $msgInfo will be a message about it, indicating the number of rows that were inserted. Otherwise, $msgInfo will acquire the value of an error message, obtained from $stmt->errorInfo()[2] . The value 2 that appears there is because errorInfo returns an array, and the error message is found in the key 2 of that array.

Optimize the connection string

Seeing the way you connect, I commented that you can optimize your connection in this way:

    $arrOptions = array(
        PDO::ATTR_EMULATE_PREPARES => FALSE, 
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, 
        PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'"
    );

    $base=new PDO('mysql:host=localhost; dbname=bdnutriologo','root',$arrOptions);

As you can see, I have created a variable $arrOptions in which I indicate how I want my connection and pass that array as the last parameter of my connection. This will prevent applying two methods to the object after it has been created .

Among the options I have put one that is very important for security: PDO::ATTR_EMULATE_PREPARES => FALSE , not put it could suppose that PDO could emulate the prepared queries and in some scenarios you could strain an SQL injection.

The discharged message comes out because it is independent of the result of the insert.

The method execute returns a True or False depending on the success of the query Why do not you try control it with that?

Edit: The specific method for errors with PDO is ErrorInfo () , on the sample page It comes out like that.

$stmt = $dbh->prepare('bogus sql');

if (!$stmt) {
    echo "\nPDO::errorInfo():\n";
    print_r($dbh->errorInfo());
}

Where $ stmt would be your $ result .