How accurate is ultrasound to date a pregnancy?

Ultrasound has become so helpful that obstetricians now refer to the time before it was used routinely as "the olden days." We use it to diagnose twins early on; we use it to document appropriate growth as the pregnancy progresses; we use it to determine fetal health; and we use it to guide conversion of breech to vertex (head-first) position and to guide amniocentesis.
Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles). Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there's a risk of delayed growth, as in hypertension or smoking, or if there's the risk of exaggerated growth, as in gestational diabetes. It is not uncommon for babies that are labeled "Large for Gestational Age (LGA)" and "IntraUterine Growth Restriction (IUGR)" to have monthly or even weekly ultrasounds during the pregnancy.
When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done. These measurements are computed automatically in the ultrasound machine's software. The software has certain measurement scales based on data from large populations, and your baby's measurements are put into this scale. In other words, by comparing your baby's measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.
However, if you have a larger than average baby, the ultrasound will apply the husky figures to the "normal" measurements. This creates error in that the baby will compute out to be further along than he or she really is. For instance, say you normally have eight and nine pound babies. Then your baby at 37 weeks will have bigger measurements than babies destined to weigh seven or eight pounds at birth. The computer in the ultrasound (ignorantly) lumps your bigger baby into the dates of babies that big in the "normal" population. This may then indicate that your 37-week baby is two weeks overdue!
Therefore, measurements taken later on, when babies begin to grow at different rates among pregnancies, yield increasingly inaccurate dating of pregnancy. Although the dating of pregnancy accuracy takes a dive, still ultrasound is indispensable for the other uses.
In early pregnancy, the measurement from the top of the fetal head to the bottom of the pelvis is called the "Crown-Rump" length. Before twelve weeks, it's difficult to measure much more. Nevertheless, this measurement is fairly accurate. In the second trimester, (past the first 12 weeks), the other measurements can be added. Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so. After that, the variations among babies' measurements cause increasing inaccuracies. The femur (thigh bone) length seems to hold on to its accuracy longer than the other parameters, but after 36 weeks it isn't foolproof.
Two ultrasounds taken one month apart that agree with each other on when the due date is, yield a very accurate and reliable answer. The simple rule in ultrasound is 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. This doesn't apply later on in pregnancy for the reasons above.

So how accurate is ultrasound?

If two ultrasounds one month apart determine coinciding due dates, especially if they agree with the mother's date based on a last period, the accuracy can be within a couple of days. But it's still just arithmetic. Babies have their own clock and can come anywhere from three weeks before this exquisitely determined due date till two weeks after. So in this arithmetic test, the baby sets the curve.