When a scan at 31 weeks says that the femur length is 30 weeks, we have to look parameters like first dating scan. If the dating scan also shows 5 days or 1 week lag, then that means there is no lag at all. Then there are other parameters like fetal weight, the other parameters of fetal growth, water around the baby. Everything has to be analysed. Usually at third trimester, these scans can be plus or minus difference. So plus or minus one week is nothing to worry when the other parameters are normal. If there are other changes and other parameters, then we have to rescan sometimes 2 -3 weeks to see the change in the growth. If there is progressive delay, then it has to be followed up.

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