Measure 3 weeks behind 😯

I'm 39, pregnancy #7, know exactly when I ovulated last so everything said I'm 13 weeks. But my 1st OB appointment was today & they did two ultrasounds by 2 different machines & says baby is measuring 10 weeks. Now due to some impossible circumstances I absolutely know it's more than 10 weeks due to certain factors. Does baby growth get hindered by my age or anything or can measurement be inaccurate? I didn't get to see the Dr after the ultrasound and don't go back for a month to ask the questions. The reason for 2 ultrasounds is I have a very very tilted uterus & they could barely find baby with the 1st machine. Is it possible that all of a sudden at the 20 weeks things balance out? Just curious if anyone has had any experiences.