Ultrasounds could be wrong, however- the earlier you get them the more accurate they are- it's totally possible to conceive 4 weeks after your period, especially if you had an irregular cycle. Not all women ovulate at exactly 14 days after their period- it's the most common, but it's totally possible to ovulate later. Go with the due date your midwife has given you, they will watch for issues as you get closer to your due date and more likely then not, your body will take care of going into labor when it's the right time. Congratulations!
People do have really long cycles, hon. I did for a good long while where I would ovulate every 30 to 40 days and even longer in between. It happens. Come back and tell us what your doctor says. Yes, congratulations. I want to hear how things are going!!