Usually we don't take gestational period in the 3rd trimester .there will be a difference of 2_3 weeks .                                                                1.are your cycles regular or irregular .                   2 . If regular , correlate with 1st trimester scan ie up to 12 weeks . usually there will be a difference of 5-6 days .                                             3.If it correlates with L.M.P. then no problem .

Do u have regular cycles? If so then ido ur earlier sonographies match with last menstrual day If all were matching , then now u have intrauterine growth retardation NST biophysical profile is must N plan for delivery

Hello dear Information incomplete Reason might b prolonged cycles or delayed conception or wrong calculation of dates