Ultrasound accuracy for dating
This is presuming that the pregnancy is actually at this stage of development.Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.I would go to an actual clinic (like an MFM) that specializes in dating ultrasounds.In general, the earliest scans are more accurate than dating scans performed later in pregnancy…The earlier the ultrasound is done, the more accurate it is at estimating the baby's due date.
Ultrasounds performed after 22 weeks gestation cannot be used to estimate the due date of the baby because the size no longer reflects the age very well.
You can calculate your due date by subtracting three months from the first day of your last menstrual period (LMP) and then add a week. A pregnancy is based on being 280 days long, which is 40 weeks (more like 10 months not 9! When we give you a due date we consider 37 to 42 weeks to be full-term, so even if your baby is born two weeks before your due date it is not considered premature.
The accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated (released an egg) on day 14 of your cycle and conceived right away - none of which may be true.
but the dates you gave and sizes are just really off.
Most dating scans are done with a trans-abdominal transducer and a fullish bladder.
For example if you a tiny sac can be seen in the uterus, but the baby and its heart beat may not be detected yet.