add_filter( 'the_content', 'add_related_post' );
function add_related_post( $content ) {
        $related_posts= "<div class='meta-related'>".do_shortcode('[related_posts]')."</div>";
        if ( is_single() ) {
                return count_p( $related_posts, 1, $content );
        }
        return $content;
}