I'm 15 weeks pregnant how big should my baby be?

No not 5.5 cm 14 cm 5.5 in

I think you mean the baby was 5.5 cm, not inches. 5 inches would be absolutely enormous for 12 weeks, and no doctor would let you walk out without doing follow up testing if you measured at 5 inches that early.

Ultrasound measurements at that stage are in centimeters, not inches.

Assuming you were 10-11 weeks at your last US, the baby should only have been measuring 1" or 1 & 1/2". So, either the measurements were misread or misinterpreted. An 11 week fetus' development is completely different than a 15 week old fetus, so having a due date be off by a month or more in an earlier US for that matter, would be highly unlikely. I'll be curious what your next US says, but I truly think there was a misinterpretation somewhere. Google an 11 week fetus and 15 week fetus. You'll see what I mean when early development should be very hard to get wrong.


Expecting 3rd

Yes, your due date could be off.
Yes, you could have an big baby.

I had an big baby, they kept moving the due date back.
I knew when I got pregnant, well at least the month and date of my last period. So we went with that due date.

Ultrasound are base on average size baby.
If you child is bigger or smaller, the due date could be changed.

If you are 100 percent sure when your last period was, you could have the wrong due date.