During pregnancy how accurate are ultrasounds to determine how many weeks and months?

In the early weeks, it's accurate within a few days, typically. As pregnancy progresses, it becomes less and less accurate in general, due to the normal variations in size and development of babies (which is determined by genetics, not a standard code). For instance, someone growing a 6lb baby is going to have a different ultrasound guesstimate than a woman growing a 9lb baby (both are totally normal weight variations) at the same gestational age after the first trimester.

The most accurate method of dating is charting your fertility. However, all have a margin of error and estimated due dates are based on an average between 38-42 weeks after the last menstrual period (or 2 weeks before ovulation, to be more precise). Although anything from 37-43 weeks is to be normally expected for a healthy, full-term baby. Beyond is typically not a problem (though can be in rare instances), but sooner is considered premature.

The earlier in the pregnancy the ultrasound, the more accurate its results. The measurements they can get early on tend to be very accurate in dating the pregnancy.

I had an ultrasound at ~14 weeks (for twins) that was considered fairly reliable. The perinatologist was pretty confident of the dates/age (and their rate of development over the pregnancy plus their birthdate and "condition" at birth confirmed the accuracy), but measurements taken earlier would have been more reliable or "certain," in terms of there being less room for error.

Ultrasound measurements (& the resulting weight estimates) in later pregnancy tend to be less accurate, and can be wrong in either direction (significantly under-estimating or over-estimating size.) Those later ultrasounds tend to be more about estimating size than dating a pregnancy, though.

Early ultrasound for the purpose of dating the gestation tend to give accurate results.

They are accurate plus or minus two weeks for sure, but can be more accurate. Also, you have to consider that some women just carry babies for longer or shorter amounts of time. One woman's child may be ready to be born at 38 weeks. Another at 42 weeks, both perfectly normal and healthy. Just as we all grew at different rates as toddlers and children, so too all babies grow somewhat differently in the womb - but usually anywhere between 38 and 42 weeks before they are fully ready to be birthed.

They're fairly accurate. It's determined by the stage of growth of the fetus. IE...have the limbs developed?