What are cookies?
Cookies are little bits of information that are sent to your web browser, which stores the information in a file. The next time you visit the site, the browser will send the cookie back to the web site. This cookie contains very limited information, in this case, it only contains your due date. No other web site can read this cookie, only amazingpregnancy.com, and only in the duedate pages.
This cookie means that when you come back to the site to view your latest calendar, you do not have to re-enter your due date each time.