Ultrasound accuracy for dating
This is because the fetus is growing so quickly that there is a big difference in size from week to week.
Also the fetus cannot bend and twist too much yet, so the length of the body is fairly constant.
At 4.5 to 5 weeks, a gestational sac may be seen in the uterus using transvaginal ultrasound, Joseph Woo, M.
Before the advent of routine ultrasound in pregnancy, medical personnel estimated a baby’s due date by the date of the last menstrual period and by feeling the uterus and determining the size to see if it matched the given due date.Most dating scans are done with a trans-abdominal transducer and a fullish bladder.