The phrase "twelfth week" refers to the period of time that comes after eleven weeks. It is a way of expressing a specific timeframe or point in a schedule or timeline. Full definition
You may register your pregnancy in hospital beginning from twelfth week, but if you are worried about something, you may do it earlier. (infobaby.org)
Janaina is starting with the recipe for the twelfth week of Spring, which includes Fried Bananas with Pecans and Maple Syrup, Fatoush Salad with Grilled Halloumi and Asparagus Risotto. (meatfreemondays.com)
Personally, I honestly felt like I had exited a cave after the twelfth week of pregnancy, as if I had been previously kidnapped and heavily sedated with only the occasional reminder I was alive from an unexpected rush of nausea. (believemidwiferyservices.com)