are you sure your not 14 weeks measuring 18 weeks instead of 41 weeks. I am a little confused because if you are 41 weeks then that means you are past your due date and that if you are measuring that small someone should have caught that way before now and there is a problem.
Now if you are only 14 weeks measuring at 18 weeks that is more understandable and could mean a couple of things
1) You are further along than you and your doctor thought so your due date is off
2) You could just have a big baby
3) You may have gestational diabetes which you will be tested for about 24-26 weeks
but can cause large babies I am not sure if the baby would be affected that early in your pregnacy though or not
Either way this is something you should discuss with your doctor soon
That you should contact your doctor.