What's up with all these celebrity twins?

Okay, can someone please tell me what they are putting in the water in Hollywood these days? Because all the stars seem to be having twins. Look at the list:

• Jennifer Lopez & Marc Anthony-twins!


• P.Diddy-twins!


• Brad & Angelina-twins!


• Julia Roberts-twins!


• Ricky Martin-twins!


• Patrick Dempsey-twins!


• 'Ugly Betty's' Rebecca Romijn-twins!


• Lisa Marie Presley-twins!

Are these all IVF babies? If so, why is no one mentioning this? Or is this all just coincidence?