Can a ultrasound not be accurate by a week or so in determining how many weeks you are pregnant?

Yes, don't worry about the difference.

By 4 weeks, a fetus cannot be seen but the thickening of the uterine lining can be detected. By around 5 weeks, an ultrasound can almost always confirm that you are pregnant.

Between four and a half to five weeks, the gestational sac is easily seen and can be measured, since the average growth is 1mm per day at this time but not completely uniform this information is considered a tentative date when LMP date is not known.

A few weeks later Ultra Sound to determine fetal age is done by taking a measurement of either the femur (longest bone in the body) or the crown of the head to the pelvis called the "crown rump". The measurement is taken and automatically then by comparing your baby's measurements to the data from a large collection of measurements, the ultrasound can then tell how far along your baby is.This is most accurate between week 7 and week 14.

Doctors have fallen into using a rule in ultrasound that when the due date based on ultrasound doesn't vary from the mother's dates by more than a week, stick with the mother's dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.

Later in pregnancy other things such as babies who are larger than average size, babies growing at different rates among pregnancies, will cause inaccurate dating of pregnancy. But are still an excellent tool for other issues that may develop.

The bottom line is, Ultra Sound is a machine made and run by humans, it is an amazing tool for many things, but babies are still going to come into this world when they are ready, regardless of dates. Only 5 to 10 percent of babies arrive on their due date, whether the date is by ultra sound or LMP.