Is it possible that a sonogram taken in the first trimester to be off by 4 or 5 weeks?

Gestational dating is all based on Best Guess from available evidence.
If there is a paternity issue then a DNA test AFTER the birth with all concerned parties would be in order.

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Originally Posted by Curlyben

Gestational dating is all based on Best Guess from available evidence.
If there is a paternity issue then a DNA test AFTER the birth with all concerned parties would be in order.

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K doesn't really answer my question... But thanks anyway.

Not usually that far off. However, remember that we are still talking about a very small body.

Ultrasounds in early pregnancy, first trimester, are usually very accurate because they are measuring the organs that develop in a specific order, and measuring from head to rump. Rarely will it be off more than 2 weeks.

Am I correct in assuming that paternity may be in question?