When do yours start?

I go once more to check baby's heart this coming Friday and then I go again after Christmas to see if my placenta I'll be 28 almost 29 weeks. They are checking then and making the call. Some people I know start going every 2 weeks at 28 weeks and then at 36 weeks it's every week. For me I'm having baby at 37 or 39 weeks. They will schedule accordingly when they check my placenta. Anyone else had this? What to expect?