###Human: The gestational age in scan is different from the weeks when calculated from my LMP. Why? ###Assistant: Hi. For a lady, with regular periods previously, generally ovulation occurs around 14th day of the cycle, so fertilization can take place only after that. In your case (if your periods are regular), you ovulated around 22nd two months back and fertilization took place only after that. So originally, the baby will always be two weeks less than what is calculated by LMP. Secondly, first-trimester USG can have an error of up to seven days while second and third trimester up to 2 and 3 weeks respectively, so do not worry if the calculation and USG are not exactly the same.