Ultrasound is a widely used and reliable method for estimating pregnancy duration. However, it is important to note that there can be a margin of error in the measurement. Generally, ultrasounds are considered to be accurate within a range of plus or minus two weeks. This means that if your ultrasound says you are 34 weeks pregnant, your actual pregnancy duration could be anywhere between 32 and 36 weeks. It is also worth mentioning that the accuracy of ultrasound measurements can vary depending on factors such as the skill of the technician and the position of the baby during the scan. In some cases, additional ultrasounds may be recommended to monitor fetal growth and ensure more accurate estimations. If you have concerns about the accuracy of your ultrasound results, it's best to consult with your healthcare provider who can provide further guidance based on your specific situation.