Ultrasound can very accurately measure the baby at any time in gestation. It cannot tell how big the child will be when it is born, but it can measure the skull, leg bones and abdominal circumference and calculate the approximate weight of the fetus at any time. Ultrasound measurements get less accurate as the baby gets bigger... In the third trimester the weight can be inaccurate by up to 20 percent, but earlier it is much more accurate. The sex can be determined by ultrasound usually between 4 and 5 months (16-20 weeks) depending on the accuracy of the machine. I hope this was helpful and good luck with your pregnancy.