Ovarian cyst

I went in today for my 1st apt. I'm just over 10 weeks. And dr just did a peak us to check heart beat and confirm how many. He noticed a ovarian cyst and just stated they were common during early pregnancy. And moved right a long. I've never had an ovarian cyst before that I know of and would like to know what about pregnancy would cause one. I googled but got no where really. Thanks in advance ladies!