Thanks for the query.
Yes, there are possibilities that there could be a lack of growth in the fetus due to chromosomal anomalies or genetic causes or infections in the first trimester that could give a lag of 3 weeks on scan compared to maternal dates. The other cause could be unreliable menstrual period date given by the mother in case her cycles have been irregular or caused due to withdrawal bleeding. We cannot ascertain the paternity on ultrasound. You will need to do DNA karyotyping tests for that.
Hope this helped.
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