Inténtalo así:
Código php:
Ver originalRewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.mipaginaweb\.com
RewriteRule (.*) http://www.mipaginaweb.com/$1 [R=301,L]
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress